text: Detection of exclusive reactions in the Hermes Recoil Fiber Tracker Inaugural-Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften a der Justus-Liebig-Universit¨t Giessen Fachbereich 07 (Mathematik und Informatik, Physik, Geographie) vorgelegt von Tibor Keri aus Subotica, Jugoslawien a II. Physikalisches Institut der Justus-Liebig-Universit¨t Giessen Juli 2008 Dekan: Prof. Dr. Bernd Baumann u 1. Berichterstatter: Prof. Dr. M. D¨ren 2. Berichterstatter: Prof. Dr. C.-D. Kohl Zusammenfassung Das Standardmodell der Teilchenphysik beschreibt die grundlegenden Be- a standteile und Kr¨fte in unserer Welt. Allerdings gehen viele Aspekte der ¨ subatomaren Welt noch uber dem Bereich der theoretischen Vorhersagen o hinaus. Dazu geh¨rt, daß die Zusammensetzung des Nukleonspins, welches a Drehimpuls- und Bahndrehimpulsbeitr¨ge von Quarks und Gluonen enth¨lt, a a a noch nicht vollst¨ndig verstanden wird. Der tats¨chliche Quarkdrehimpuls- a a beitrag zum vollst¨ndigen Nukleonspins betr¨gt nur etwa 30%. Ein neuen- o twickelter Formalismus erm¨glicht die interne Struktur des Nukleons durch so genannte GPDs (Generalized Parton Distributions) auf eine umfangre- ichere Art als die vorher verwendeten PDFs (Parton Density Functions) zu beschreiben. Diese GPDs werden durch die Summenregel von Ji mit a den Beitr¨gen des Gesamtdrehimpulses von Quarks und Gluonen verkn¨pft u o und k¨nnen durch die Untersuchung von harten exklusiven Reaktionen bes- timmt werden. DVCS (deeply virtual Compton scattering) ist die einfachste exklusive Reaktion, um unter Verwendung von Leptonenstrahlen mit ver- a schiedenen Helizit¨ten und Ladungen einige dieser Verteilungen zu messen. HERMES (HERA measurements of spin) ist eines der Experimente zur Untersuchung des Nukleonenspins. Es befindet sich innerhalb HERA (Hadr- onen-Elektronen Ring Anlage), welches ein e± -p-Beschleuniger am DESY (Deutsches Elektronen-Synchrotron) ist. Bei dem HERMES Experiment wird nur der polarisierte Elektron-/Positron-Strahl verwendet, welcher an wahlweise polarisierten oder unpolarisierten Gasen gestreut wird. Das Vor- a w¨rtsspektrometer vom HERMES Experiment besteht aus Spurdetektoren sowie Detektoren zur Teilchenidentifikation. In der ersten Phase der Daten- a nahme wurden nur die Teilchen in Vorw¨rtsrichtung detektiert. Die Kine- matik von exklusiven Reaktionen wurden unter der Verwendung fehlender a invarianter Restmasse vervollst¨ndigt. a o Um die Exklusivit¨t zu verbessern und um die Aufl¨sung kinematis- o a cher Variablen zu erh¨hen, wurde das HERMES Vorw¨rtsspektrometer um den RD (Recoil Detector) erweitert, wobei danach die Messungen mit un- polarisierten Gasen erfolgten. Dieser Detektor besteht aus dem Silicon Strip Detector, dem SFT (Scintillating Fiber Tracker), dem Photon De- tector und wird von einem supraleitenden Magneten mit einer Feldst¨rke a u von 1T umgeben. Er stellt mehrere Raumpunkte f¨r Spurrekonstruktion u u und damit f¨r die Impulsrekonstruktion zur Verf¨gung. Die Energieverluste der Teilchen beim Passieren der verschiedenen Detektoren werden verwen- o det, um die Teilchen identifizieren zu k¨nnen. Der Hauptteil dieser Arbeit war die Implementierung des SFT- und des RD-Auslesesystems. Vor der Installation des RD wurde eine Reihe von Messungen mit ver- u schiedenen Versuchsaufbauten durchgef¨hrt, um das Konzept des Detektors u zu pr¨fen, die interne Ausrichtung zu messen und die Installation vorzubere- u iten. Diese Messungen f¨r den SFT werden beschrieben und die wesentlich- en Resultate werden gezeigt. Ausserdem wurde eine erste Analyse der ak- u tuellsten Datenproduktion 06d/06d0 durchgef¨hrt, um die Leistung des in- a stallierten RD in Verbindung mit dem HERMES Vorw¨rtsspektrometer zu zeigen. Abstract The standard model of particle physics describes successfully the funda- mental constituents and forces in our world; nevertheless, many details of the subatomic world are still beyond the scope of theoretical predictions. The internal structure of the nucleon has been investigated in detail and it was found that the nucleon spin budget, i.e. the composition of the nu- cleon spin by the spin and orbital angular momentum of quarks and gluons is not yet understood. It has been measured that the intrinsic quark spin contribution is only about 30% of the total spin of the nucleon. A recently developed formalism allows to describe the internal structure of the nucleon by so-called GPDs (Generalized Parton Distributions) in a more complete way than the previously used PDFs (Parton Density Functions). The GPDs are linked by the Ji sum rule to the angular momentum contributions of quarks and gluons. These GPDs can be accessed by the investigation of hard exclusive reactions. DVCS (deeply virtual Compton scattering) is the cleanest exclusive reaction to determine some of these distributions, using lepton beams with different helicity states and charges. HERMES (HERA measurements of spin) is one of the experiments which were carried out to complete the information about the nucleon spin budget. It is located at HERA which is an e± -p-collider at DESY but uses only the polarized electron- and positron-beam, which is scattered off a gaseous internal target. The HERMES forward spectrometer consists of a set of detectors that are used for tracking, while another set of detectors provides information on particle identification and triggering. In the first phase of HERMES, only forward going particles were detected. Exclusive reactions have been measured using a missing invariant mass technique. In order to improve exclusivity and to enhance the resolution of kine- matic variables the HERMES collaboration decided to remove the equip- ment for the polarized target and to install the RD (Recoil Detector) with an unpolarized target at this position. This detector consists of the Silicon Strip Detector, the SFT (Scintillating Fiber Tracker), the Photon Detector and is surrounded by a 1T superconducting magnet. It provides several space points for tracking and thus momentum reconstruction. The energy deposition in the various detectors is used to achieve particle identification. The main part of the thesis work was the implementation of the SFT and the RD readout system. Before the installation of the RD a series of test runs were carried out to proof the concept of the detector, to measure the internal alignment and to prepare the installation. These test runs for the SFT are described and major results are shown. Furthermore a preliminary analysis of the latest data 06d/06d0 was carried out to show the performance of the installed Recoil Detector in combination with the HERMES forward spectrometer. Contents 1 Introduction 8 2 Nucleon spin budget 10 2.1 Deep Inelastic Scattering . . . . . . . . . . . . . . . . . . . . . 10 2.2 Generalized Parton Distributions . . . . . . . . . . . . . . . . 13 2.3 Deeply Virtual Compton Scattering . . . . . . . . . . . . . . 16 2.3.1 Azimuthal asymmetry . . . . . . . . . . . . . . . . . . 18 2.4 Results at HERMES . . . . . . . . . . . . . . . . . . . . . . . 19 2.4.1 Beam Spin Asymmetry . . . . . . . . . . . . . . . . . 20 2.4.2 Beam Charge Asymmetry . . . . . . . . . . . . . . . . 21 2.4.3 Transverse Target Spin Asymmetry . . . . . . . . . . 22 3 The HERMES Experiment 25 3.1 HERA at DESY . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2 Target system . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.3 Forward spectrometer . . . . . . . . . . . . . . . . . . . . . . 31 3.3.1 Luminosity monitor . . . . . . . . . . . . . . . . . . . 32 3.3.2 Magnet system . . . . . . . . . . . . . . . . . . . . . . 34 3.3.3 Tracking system . . . . . . . . . . . . . . . . . . . . . 35 3.3.4 Particle identification . . . . . . . . . . . . . . . . . . 38 4 The Recoil Detector 47 4.1 Design requirements and realization . . . . . . . . . . . . . . 48 4.2 Target cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.3 Silicon Strip Detector . . . . . . . . . . . . . . . . . . . . . . 53 4.4 Scintillating Fiber Tracker . . . . . . . . . . . . . . . . . . . . 55 4.4.1 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.4.2 Detection of light by MAPMT . . . . . . . . . . . . . 59 4.4.3 Analog to Digital Converter . . . . . . . . . . . . . . . 60 4.4.4 ADC Readout . . . . . . . . . . . . . . . . . . . . . . 62 4.5 Photon Detector . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.6 Superconducting Magnet . . . . . . . . . . . . . . . . . . . . . 66 4.7 Installation of the Recoil Detector . . . . . . . . . . . . . . . 69 6 7 5 Performance 71 5.1 Proof of concept . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.1.1 Test beam environment . . . . . . . . . . . . . . . . . 71 5.1.2 Detector prototype . . . . . . . . . . . . . . . . . . . . 73 5.1.3 Test beam setup . . . . . . . . . . . . . . . . . . . . . 73 5.1.4 Test beam results . . . . . . . . . . . . . . . . . . . . . 73 5.2 Alignment run . . . . . . . . . . . . . . . . . . . . . . . . . . 78 5.2.1 Environment at test beam 22 . . . . . . . . . . . . . . 78 5.2.2 Setup of the alignment run . . . . . . . . . . . . . . . 78 5.2.3 SFT readout in 'run mode' . . . . . . . . . . . . . . . 78 5.2.4 Alignment run results . . . . . . . . . . . . . . . . . . 79 5.3 Cosmic ray test run . . . . . . . . . . . . . . . . . . . . . . . 83 5.3.1 Readout of the SFT . . . . . . . . . . . . . . . . . . . 83 5.3.2 Recoil Detector readout . . . . . . . . . . . . . . . . . 85 5.3.3 Superconducting magnet . . . . . . . . . . . . . . . . . 86 5.3.4 Cosmic ray test run results . . . . . . . . . . . . . . . 87 5.4 HERMES experiment run . . . . . . . . . . . . . . . . . . . . 88 5.4.1 Production . . . . . . . . . . . . . . . . . . . . . . . . 89 5.4.2 Overview of elastic scattering events . . . . . . . . . . 92 5.4.3 Overview of Deep Inelastic Scattering events . . . . . 92 5.4.4 Overview of DVCS events . . . . . . . . . . . . . . . . 95 5.4.5 Overview of Azimuthal Asymmetry . . . . . . . . . . . 98 5.4.6 Overview of SFT response . . . . . . . . . . . . . . . . 101 5.4.7 Overview of improvements by the Recoil Detector . . 101 5.4.8 Conclusion and Outlook . . . . . . . . . . . . . . . . . 103 6 Summary 108 A Data formats 116 A.1 Data format for standalone readout . . . . . . . . . . . . . . . 116 A.1.1 Node structure . . . . . . . . . . . . . . . . . . . . . . 116 A.1.2 Sequence of nodes . . . . . . . . . . . . . . . . . . . . 117 A.2 Data format for experiment readout . . . . . . . . . . . . . . 123 A.2.1 Silicon Strip Detector . . . . . . . . . . . . . . . . . . 123 A.2.2 Scintillating Fiber Tracker . . . . . . . . . . . . . . . . 123 A.2.3 Photon Detector . . . . . . . . . . . . . . . . . . . . . 126 Bibliography 128 List of Figures 137 List of Tables 140 Acknowledgements 141 Chapter 1 Introduction The microscopic world can be very successfully described by the atomic model developed by Bohr [Boh13] and confined by Sommerfeld [Som15]. In this model atoms are constituted by protons, neutrons and electrons. The subatomic world described by the standard model of particle physics is not yet completely clarified. The common agreement until now is the point like property of the electrons while the protons and neutrons seems to be com- pound objects. These components are quarks with gluons as carrier of the strong forces. Further investigations condensate into the well known stan- dard model [Gri87] containing QCD (Quantum Chromo Dynamics) where quarks and leptons are the constituents of matter while the electromagnetic, strong and weak forces are interacting by the carrier photons, gluons and bosons. Even the description of matter and the predictions are very well tested this model is not though yet complete. The aspect of gravity is still remaining to be taken into account and calculations on long distance scale are challenging and not done yet analytically. Mainly the complete nucleon spin budget is still unclear until now as well. Starting with elastic scattering the transverse spatial distribution of the partons can be described in the frame of quantum electro dynamics by FFs (Form Factors) very precisely. By investigations of deep inelastic scattering in the frame of quantum chromo dynamics the longitudinal momentum dis- tributions are accessible and can be described by PDFs (Parton Distribution Functions). The uncertainty relation limits the simultaneously measurable resolutions of corresponding properties like location and momentum. As FF describing transverse spatial distributions and PDF describing longitu- dinal momentum distributions both properties are disentangled. They can be combined simultaneously to distributions named GPDs (Generalized Par- ton Distributions) [Mue+94, Ji97a] which was developed in the recent years. Beside combining the both models together the Ji [Ji97b] sum rule opens with GPDs several new paths to access further informations about the na- ture of the nucleon. Several GPDs can be accessed by investigations of hard 8 CHAPTER 1. INTRODUCTION 9 exclusive reactions. Therefore GPDs are more and more used for studies. Many experiments have been carried out to identify the contributors to the nucleon spin and to investigate their constraints. A naive quark model where three quarks as partons of the nucleon carry the whole spin of the nu- cleon was a first try to explain the nucleon spin budget. This simple model has to be refined as measurements by the EMC (European Muon Collabora- tion) [Ash+88, Ash+89] show that the intrinsic contribution by quarks are much lower than expected. In order to solve this so called spin crisis several high energy experiments were started to search answers to these contribu- tions. One of these experiments is the HERMES (HERA measurements of spin) experiment [Her+93, Due95] which was running inside HERA (Hadron Elektron Ring Anlage) at DESY (Deutsche Elektronen Synchrotron). The HERMES experiment was designed to contribute information about the nu- cleon spin budget. Due to improvements in understanding of deep inelastic scattering and new models like GPDs the physics program has been widely extended. The HERMES experiment was running with electron or positron beam. The target was either polarized gas of 1 H, 2 H, 3 He or unpolarized gas of 4 He, N , N e, Kr, Xe. The HERMES detector was built as a forward spectrometer for quasi fixed target which consists of identical upper and lower half. A set of detectors is used to reconstruct the particle track and to determine the particle momentum. Another set of detectors is used for particle identification. As the space in front of the HERMES detector was occupied by the equipment of the polarized gas target the recoiling particles were not detectable. The complete kinematics of the exclusive reactions was computed with the missing invariant mass technique. To improve exclusivity and the resolution of kinematic variables the HERMES collaboration decided to upgrade the HERMES detector with the Recoil Detector. The aim of this new Recoil Detector is directly detect recoiling particles and measure their properties. The Recoil Detector con- sists of a Silicon Strip Detector, a Scintillating Fiber Tracker and a Photon Detector. It is surrounded by a superconducting magnet with a 1T center field strength. The core of the this work was the implementation of the readout for the Scintillating Fiber Tracker and to bind the readout for the Recoil Detector to the HERMES data acquisition. The outline of this work starting with the motivation why measurements of hard exclusive reactions are attractive. Afterwards the HERMES forward spectrometer with the used facilities is described. Hereafter the Recoil De- tector with the major components is recorded as well. Rearwards the results of several test runs for the preparation of the Recoil Detector and some pre- liminary results of the experiment are presented. The results of the 06d/06d0 production are compared to previous results. Chapter 2 Nucleon spin budget The standard model of particle physics is perhaps the best proven model. Nevertheless the nucleon spin budget is not yet completely clarified. A pos- sible path to access informations about the constituents and their contri- butions is the recently developed GPDs (Generalized Parton Distributions) which can be ascertained by investigations of exclusive reactions. In front of the description a short introduction of the nomenclature to describe prop- erties of scattering reactions and at the end recent results at HERMES are shown. The particles of the electron-/positron-beam are named lepton in this work. 2.1 Deep Inelastic Scattering The general process visualized in figure 2.1 for a lepton scattering reaction can be expressed by n ′ (2.1) k+N →k + Xj . j=1 with N as the nucleon in the initial state, k (k′ ) as the lepton in the inital (final) state and where n denotes the number of the scattered and produced particles. In the limit of inelastic scattering, for an elastic scattering process the nukleon N remains intact, thus follows n = 1 and X1 = N . Further categorizations are inclusive, semi-inclusive and exclusive mea- surements. In case of an inclusive measurement only the scattered lepton is detected, while in semi-inclusive measurements some but not all other particles are detected. If all particles in the final state are detected the measurement is called exclusive. The HERMES experiment [Her+93, Due95] uses as will be shown later a 27.6GeV polarized lepton beam. In this case other interactions like weak forces at higher energies can be neglected. 10 CHAPTER 2. NUCLEON SPIN BUDGET 11 Figure 2.1: Illustration of a deep inelastic scattering reaction where the incoming lepton (k) scatters off and interacts with the nucleon by a virtual photon. The four momentum transfered by the virtual photon is the difference (2.2) q = k − k′ of the four momentum of the incoming k and scattered k′ lepton. The square of the four momentum transfer q due to the uncertainty relation can be interpreted as a spatial resolution Q2 ∼ λ−2 of the probing and can be expressed as (2.3) Q2 = −q 2 . The value q is a space like property and therefore q 2 is negative. In order to have a positive resolution the negative square of q is used as the spatial resolution Q2 . The energy ν transfered by the virtual photon can be expressed as p·q (2.4) ν= M CHAPTER 2. NUCLEON SPIN BUDGET 12 where M denotes the nucleon mass and where p is the four momentum of the nucleon before the interaction. The squared sum of the four momentums of the initial nucleon and the incoming lepton holds the inital state invariant mass (2.5) s = (p + k)2 of the reaction. The squared sum of the four momentum of the nucleon and the inter- acting virtual photon results to the invariant mass (2.6) W 2 = (p + q)2 of the nucleon-photon-system. The Bjoerken work [Bjo69] shows that the description of interactions can be made without properties with physical dimensions. Therefore the following properties are dimensionless and are interpreted accordingly. The fractional energy y of the virtual photon can be defined as p·q (2.7) y= . p·k The four momentum fraction x of the nucleon-photon-system to the center-of-mass system can be written as −q 2 Q2 (2.8) x= = . 2p · q 2p · q As W 2 can be expressed as (2.9) W 2 = (p + q)2 = p2 + 2p · q + p2 = p2 + 2p · q (1 − x) x can be interpreted as a measure of the inelasticity of the process as in an elastic process the equation x = 1 holds. Due to the kinematics in inelastic processes the ranges are limited to 0 < x < 1 and to 0 < y < 1. The properties Q2 , s, W 2 , y, x are invariant against Lorentz transfor- mations. In the kinematic range of the HERMES experiment running with 27.6GeV lepton beam the rest mass of the imprinted lepton with ∼ 0.5M eV on the first order can be neglected. Under this assumption these properties can be expressed in the laboratory frame as: θ (2.10) Q2 = −q 2 ≈ 4EE ′ sin2 2 CHAPTER 2. NUCLEON SPIN BUDGET 13 p·q (2.11) ν= ≈ E − E′ M (2.12) s = (p + k)2 ≈ 2M E + M 2 (2.13) W 2 = (p + q)2 ≈ M 2 + 2M ν − Q2 p·q ν (2.14) y= ≈ p·k E Q2 Q2 Q2 (2.15) x= = ≈ 2p · q 2M ν 2M (E − E ′ ) In these equations E and E ′ denotes the energy of the incoming and scattered lepton, θ is the polar angle between the vectors of the unscattered and the scattered lepton. All these properties can be determined by the knowledge of the initial state and by detecting only the scattered lepton. 2.2 Generalized Parton Distributions GPDs (Generalized Parton Distributions) [Mue+94, Ji97a] have been devel- oped few years ago to combine transverse charge distribution information named FFs (Form factors) with longitudinal momentum distribution infor- mation named PDFs (Parton Density Functions) for description of exclusive processes. After the correlation between these GPDs and the total angular momentum was investigated by Ji [Ji97b] this description got more and more used. A very detailed description of GPDs and the current development can be found in [Die03]. GPDs consist of a set of distributions for quarks and gluons for each flavor and helicity state. Most often discussed are the four twist-2 quark- chirality conserving distributions: the polarization-independent distribu- x x tions are denoted by H (¯, ξ, t) and E (¯, ξ, t), while the polarization-depend- ent distributions are named H x ˜ x ˜ (¯, ξ, t) and E (¯, ξ, t). The distributions E and E˜ do not conserve the nucleon helicity but the distributions H and H ˜ do. The distributions of all flavors for each quark and gluon are superposed ¯ to describe the total response. The used parameters x, ξ and t are shown in figure 2.2. The four momentum p ¯ CHAPTER 2. NUCLEON SPIN BUDGET 14 (x+ξ) p (x-ξ) p GPD(x,ξ,t) p t p' ¯ Figure 2.2: Description of GPDs by momentum fraction x, skewness ξ and momentum transfer t as described in the text. p, p ′ and p are the four ¯ momentum of the target for the initial state, finial state and the averaged case. 1 (2.16) p= ¯ p + p′ 2 is the average of the four momentum p of the nucleon in the initial state and the four momentum p′ of the nucleon in the final state. The parameter x is ¯ the average longitudinal momentum fraction of the quark, while the skew- ness parameter ξ represents the fractional longitudinal-momentum transfer ¯ ¯ to that quark. Positive values of x + ξ and x − ξ are interpreted as quarks, while negative values are treat as anti-quarks. The Mandelstam variable t 2 (2.17) t = p − p′ is the squared momentum transfer to the nucleon and is therefore invariant against Lorentz transformations. x x Knowing unpolarized GPDs H (¯, ξ, t) and E (¯, ξ, t) the Ji sum rule [Ji97b] +1 1 (2.18) Jq,g = lim x¯ x x d¯ x (Hq,g (¯, ξ, t) + Eq,g (¯, ξ, t)) t→0 2 −1 CHAPTER 2. NUCLEON SPIN BUDGET 15 can be used to determine the total angular momentum contribution by quarks and gluons which is one of the key feature of this model. By knowing the intrinsic spin contributions of quarks and gluons the contribution of the orbital angular momentum can be accessed as well. As further advantage the GPDs formalism still provides access to FF ¯ and PDF. The FF are accessible by the first momentum of x while the PDF are the limit for vanished momentum transfer t → 0 to the nucleon. Exclusive reactions originated by a virtual photon γ ∗ to a nucleon p visualized in figure 2.3 can be formulated by k' γ ,ρ0,π0,... k γ* q q' (x+ξ)p (x-ξ)p GPD(x,ξ,t) p t p' Figure 2.3: Illustration of exclusive reactions by scattering of lepton (k) on a nucleon (p). (2.19) γ ∗ + p → X + p′ where the nucleon p, p′ is still intact but a different particle X is generated. These exclusive reactions are an important tool to access the properties of GPDs and therefore to the nucleon constituents. CHAPTER 2. NUCLEON SPIN BUDGET 16 Another feature of exclusive reactions is the possibility to probe certain reactions by final state selection. Final states with a ρ0 , the scattered pro- jectile and the intact nucleon allow studies of quarks and gluons on the same order and on linear order as described in [Goe+01, Die+04b], with recent results in [Ros+07] and in HERMES paper which is in preparation. But the cleanest way to access GPDs is deeply virtual Compton scatter- ing which will be introduced in the following section. 2.3 Deeply Virtual Compton Scattering Theoretically simplest process to access GPDs is DVCS (deeply virtual Compton scattering). The DVCS process visualized in figure 2.4 can be expressed by the reaction k' k γ* γ q q' p p' Figure 2.4: DVCS (deeply virtual Compton scattering. The virtual photon originated from the scattered projectile probes the nucleon which stays intact in the final state and produces a real photon. (2.20) k + p → k ′ + γ ∗ + p → k ′ + γ + p′ with k (k′ ), p (p′ ), γ ∗ , γ which are the initial (final) lepton, the inital (final) CHAPTER 2. NUCLEON SPIN BUDGET 17 nucleon, the originating virtual photon and the produced real photon. The generated real photon carry information about the involved quark as it is not generated by Bremsstrahlung. The Mandelstam variable t 2 (2.21) t = p − p′ = (q − q′)2 of the squared momentum transfer to the nucleon can be calculated alter- natively by the momentum of the two incident photons. In this case the momentum transfer can be determined without the detection of the recoiled nucleon. The final state of the DVCS process can not be distinguished from the BH (Bethe-Heitler) process as it has the same initial and final state as shown in figure 2.5. The BH process is an elastic scattering process and can be precisely calculated in the frame of QED (Quantum Electro Dynamics). e e' e e' γ γ γ* γ* p p' p p' Figure 2.5: The Bethe-Heitler process can not be distinguished in the initial and final states from the deeply virtual Compton scattering. The combined cross section [Bel+02] dσ α3 xy em τ 2 (2.22) = dxdyd|t|dφ 8πQ2 1 + 4x2 M 2 /Q2 e3 results to an interference of the amplitudes by (2.23) |τ |2 = |τBH + τDV CS |2 = |τBH |2 + |τDV CS |2 + I with the interference term CHAPTER 2. NUCLEON SPIN BUDGET 18 ∗ ∗ (2.24) I = τBH τDV CS + τDV CS τBH . The electromagnetic fine structure constant αem and the elementary charge e is used here. An additional parameter for separation between DVCS and BH is the angle θγ ∗ γ between the trajectories of the virtual photon and the produced real photon. A further parameter for describing azimuthal asymmetries is the azimuthal angle φ between the scattering plane defined by the incident and scattered lepton and the production plane defined by the momentum of the virtual and real photon shown in figure 2.6. Figure 2.6: Diagram of the kinematics for DVCS reactions. The scattering plane is defined by the trajectories of the projectile while the production plane is defined by the virtual and real photons. The angle φ is formed by these two planes. The angle θγ ∗ γ are formed by trajectories of the virtual and real photon. 2.3.1 Azimuthal asymmetry The two pure terms and the one interference term of the combined amplitude of BH-process and DVCS-process can be evolved in leading order to a Fourier sum [Bel+02] in which φ denotes the angle between the scattering plane and the production plane as shown in figure 2.6: (2.25) 2 e6 |τBH |2 = cBH + 0 cBH cos (nφ) n xy 2 (1 + 4x2 M 2 /Q2 ) P1 (φ) P2 (φ) n=1 CHAPTER 2. NUCLEON SPIN BUDGET 19 (2.26) 2 e6 |τDV CS |2 = cDV CS + 0 cDV CS cos (nφ) + λsDV CS sin (nφ) n 1 y 2 Q2 n=1 (2.27) 2 2 −C e6 I= cI + 0 cI cos (nφ) + λ n sI sin (nφ) n xy 3 tP1 (φ) P2 (φ) n=1 n=1 with C as the charge and λ as the helicity of the beam lepton, P1 (φ) and P2 (φ) as the BH propagators [Bel+02, Jaf96]. By combining measurements with different states like charge and polar- ization of the striking beam lepton certain coefficients can be determined by azimuthal asymmetries. As shown in figure 2.7 in the HERMES kinematics the amplitude of the BH process is almost over the whole θγ ∗ γ range much larger than the amplitude of the DVCS process [Kor+02]. As the contribution by the pure BH term can be precisely calculated in QED and the contribution by the pure DVCS term is of sub leading order the interference term provides an unique access to the DVCS amplitude in linear order which is significantly increased by the larger BH amplitudes compared to the pure DVCS term. The nomenclature for the coefficients of azimuthal asymmetries are ABT where B and T are the polarization of the beam and the target. These states can be L, T , U which are the cases for longitudinal, transversal or unpolarized polarization. Despite of this notation the coefficient for the beam charge asymmetry is denoted as AC . As the DVCS process provides access to all GPDs of quarks the total angular momentum contribution of the quarks to the nucleon spin budget can be determined with the Ji sum rule [Ji97b]. However the measurements for all the manifold states and parameters are still challenging. 2.4 Results at HERMES At the HERMES experiment in first years of running the recoiling particle was not detectable. The complete kinematics of the exclusive reactions are computed by the missing invariant mass technique. The newly installed Recoil Detector into the HERMES experiment detects the recoiling particles directly. This enhances exclusivity and the resolution of kinematic variables. All measured results until now are using data samples before the instal- lation of the Recoil Detector. An extended introduction and presentation of the recent results can be found in [Air+01, Ell03, Ell07, Kra05, Ye07, Air+06, Air+08]. An simple overview about results at HERMES will be CHAPTER 2. NUCLEON SPIN BUDGET 20 Figure 2.7: The solid line is the sum of the cross section sum of BH- process (dashed-dotted line) and DVCS (dashed line) versus the angle θγ ∗ γ each in the kinematic region of HERMES. θγ ∗ γ is the angle between virtual and real photon [Kor+02]. Positive values of θγ ∗ γ correspond to φ = 0, while negative ones are for φ = π. The range used to select exclusive events are 5mrad(≈ 0.28o ) < |θγ ∗ γ | < 45mrad(≈ 2.6o ) given below. The following measurements are done with unpolarized hydro- gen as a target and with an electron and a positron beam. 2.4.1 Beam Spin Asymmetry In order to determine the coefficient for the BSA (beam spin asymmetry) the measurements of longitudinal polarized lepton beam on unpolarized hy- drogen are combined and the asymmetry [Bel+02, Air+01] is given by dσ + − dσ − 1 N+ − N− x sI 1 (2.28) ALU = = ∝ C BH sin (φ) dσ + + dσ − < |PB | > N + + N − y c0 CHAPTER 2. NUCLEON SPIN BUDGET 21 where C denotes the charge of the projectile while dσ + and dσ − are the cross sections for the case of parallel and anti-parallel longitudinal beam helicity. The event counts N + and N − for the corresponding states are normalized to the luminosity and the amplitude is adjusted by the mean beam polarization < |PB | >. The result for the BSA amplitude is shown in figure 2.8. ALU 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -3 -2 -1 0 1 2 3 φ (rad) Figure 2.8: Beam spin asymmetry for electro-production of photons as function of the azimuthal angle φ [Air+01]. The dashed curve represents a sin (φ) dependence with an amplitude of 0.23, while the solid curve repre- sents the result of a model calculation taken from [Kiv+01]. The horizontal error bars represent the bin width, and the error band below represents the systematic uncertainty. 2.4.2 Beam Charge Asymmetry The BCA (beam charge asymmetry) is only measurable at HERA due to the availability of electron and positron beam. By combining measurements CHAPTER 2. NUCLEON SPIN BUDGET 22 with positrons and electrons as a projectile the leading coefficient for the beam charge asymmetry [Bel+02], [Air+06] can be expressed by dσ + − dσ − N+ − N− x cI 1 (2.29) AC = = + ∝ − BH cos (φ) dσ + + dσ − N + N− y c0 where the sign denotes the corresponding beam charge, dσ + and dσ − are the cross sections, N + and N − are the normalized to the luminosity event counts. In this case the dominant contribution is the cos (φ) part of the BCA amplitude which is shown in figure 2.9. AC 0.2 ± ± HERMES e p → e ' γ X (Mx< 1.7 GeV) 0.1 0 -0.1 P1 cos φ -0.2 P1 + P2 cos φ + P3 cos 2φ + P4 cos 3φ 0 0.5 1 1.5 2 2.5 3 |φ| (rad) Figure 2.9: Beam charge asymmetry AC as function of the azimuthal angle |φ|. The four parameter fit (−0.011 ± 0.019) + (0.060 ± 0.027) ∗ cos (φ) + (0.016 ± 0.026) ∗ cos (2φ) + (0.034 ± 0.027) ∗ cos (3φ) shown as solid line represents the total contribution of higher harmonics while the dashed line contains only the pure cos (φ) amplitude for exclusive sample before back- ground correction. Only statistical uncertainties are shown here[Air+06]. 2.4.3 Transverse Target Spin Asymmetry Electroproduction of real photons shown in figure 2.10 on transverse polar- ized hydrogen gas target allow to access total angular momentum of quarks in a model-dependent way [Air+08]. CHAPTER 2. NUCLEON SPIN BUDGET 23 S⊥ ′ k k φS q q′ φ Figure 2.10: Momenta and azimuthal angles for exclusive electroproduc- tion of photons in the target rest frame following the Trento conventions → − −→ → [Bac+04]. The quantity − = k − k ′ is the three-momentum of the virtual q → − → − photon as difference of the initial ( k ) and the final ( k ′ ) beam lepton three- → − →′ momenta, while q ′ = − denotes the three-momentum of the real photon. q The angle between the scattering plane of the lepton to the production plane of the photons is named φ, while the angle between the scattering plane of → − the lepton and the transverse target polarization S ⊥ is called φS . Measurements with both target polarities and both beam charges are used to extract azimuthal asymmetries. Beside the BCA amplitude the TTSA (transverse target spin asymmetry) amplitude is needed, which can be expressed in terms of cross sections as (2.30) 1 [dσ + (φ, φS ) − dσ + (φ, φS + π) ] − [dσ − (φ, φS ) − dσ − (φ, φS + π) ] AU T (φ, φS ) = S⊥ [dσ + (φ, φS ) + dσ + (φ, φS + π) ] + [dσ − (φ, φS ) + dσ − (φ, φS + π) ] where the dσ + and dσ − are for the corresponding beam charge and is ad- justed by the transverse target polarization S⊥ . By comparing calculations of several GPD-models with the experimentally extracted azimuthal asym- metry amplitudes of BCA and TTSA, a model-dependent constraint on the total angular momenta contribution of u- and d-quarks can be accessed [Ell+05]. Model-dependent constraints on the u-quark total angular mo- mentum Ju vs d-quark total angular momentum Jd , obtained by comparing DVCS experimental results and theoretical calculations are shown in figure 2.11. CHAPTER 2. NUCLEON SPIN BUDGET 24 Ju 1 HERMES DD (VGG) 0.5 DFJK QCDSF LHPC 0 HERMES Dual (GT) -0.5 JLab DD (VGG) -1 -1 -0.5 0 0.5 1 Jd Figure 2.11: Model-dependent constraints on u-quark total angular momen- tum Ju vs d-quark total angular momentum Jd , obtained by comparing DVCS experimental results and theoretical calculations. The constraints based on sin(φ−φS ) cos(φ) the HERMES data for the TTSA interference amplitudes AU T sin(φ−φS ) and AU T use the double-distribution (HERMES DD) [Van+99, Goe+01] or dual-parameterisation (HERMES Dual) [Guz+06] GPD mod- els. The additional band (JLab DD) is derived from the comparison of the double-distribution GPD model with neutron cross section data [Maz+07]. Also shown as small (overlapping) rectangles are results from lattice gauge theory by the QCDSF [Bro+07] and LHPC [Hae+07] collaborations, as well as a result for only the valence quark contribution (DFJK) based on zero- skewness GPDs extracted from nucleon form factor data [Die+04a, Kro07]. The sizes of the small rectangles represent the statistical uncertainties of the lattice gauge results, and the parameter range for which a good DFJK fit to the nucleon form factor data was achieved. Theoretical uncertainties are unavailable. More details can be found in [Air+08]. Chapter 3 The HERMES Experiment The HERMES (HERA measurements of spin) experiment [Her+93], [Due95] was started to study the nucleon spin budget with DIS (Deep Inelastic Scat- tering). An lepton beam with the energy of 27.6GeV was used to scatter off a quasi fixed target provided by polarized gas target. The HERMES experiment was originally designed to measure only the nucleon spin budget but in the meantime the physics program was widely expanded due to im- provements in understanding of DIS and to recently developed models. The HERMES forward spectrometer provides detection of products generated by inclusive and semi-inclusive processes directly. 3.1 HERA at DESY HERA is an e-p-collider and the major facilities used by the HERMES ex- periment will be presented. DESY DESY (Deutsche Elektronen-Synchrotron) of the Helmholtz-Gemeinschaft is a research facility located in Hamburg / Germany. It is shown in figure 3.1. It was founded in 1959 with the main purpose to do scientific fundamental research which covers three major topics: • development, construction and operation of particle accelerators; • investigations of matter and force on the scale of particles; • providing synchrotron radiation for many other disciplines. Since this time a lot of pioneering experiments and technologies were carried out. Most of the accelerators are shown in figure 3.2. The first synchrotron ring DESY was built between 1960 and 1964. DORIS (Doppel-Ring-Speicher) 25 CHAPTER 3. THE HERMES EXPERIMENT 26 Figure 3.1: The laboratory at DESY in Hamburg / Germany. The picture shows the location and dimension of HERA and PETRA by the two dashed circles. See also sketch 3.2 for more details. is a facility for studies with synchrotron radiation. The PETRA (Positron- Electron Tandem Ring Accelerator) which was built between 1976 and 1978 achieved the pioneering work to discover gluons in 1979 [Wii+79]. Later in 1990 HERA (Hadronen Electronen Ring Anlage) was available to provide two accelerators combined with storage rings with a proton and a lepton beam for unique studies on particle physics. In last years pioneering work was done for TESLA (Tera Scale Accelerator) to proof the concept of next generation of linear lepton colliders as the storage rings for leptons for higher energy getting very challenging and very expensive. Recently the XFEL (X-ray Free Electron Laser) project was started to provide unique facility to measure in situ chemical reactions, to investigate atomic details of molecules or to make spatial pictures of nano scale physics. After the retirement of HERA in 2007 which will be not used for the next years the upgrade of PETRA was started to improve the properties to be the worldwide most brilliant synchrotron radiation source with additional 14 beam lines with up to 30 experimental stations installed until 2009. CHAPTER 3. THE HERMES EXPERIMENT 27 Figure 3.2: A set of pre accelerators are used to feed HERA with protons and leptons. The arrows indicates the accelerated particles flow which are marked as blue for protons and red for leptons. Four experiments were located at HERA. As indicated H1 in the north and ZEUS in the south were collider's. HERA-B in the west used only the proton beam while HERMES in the east used only the lepton beam. HERA Hera consists of two concentric rings with a circumference of around 6300m. Each ring had the facility to accelerate particles and store them for several hours. The one ring provided in the early years an unpolarized proton beam with an energy of 880GeV . After an upgrade in 1998 the energy was in- creased to 920GeV . In the last few months of HERA run time the energy of the proton beam was changed to 460GeV and 575GeV for additional studies with these lower energies. A typical value for the beam current is around 110mA at the beginning of the a fill with a life time of several hundred CHAPTER 3. THE HERMES EXPERIMENT 28 hours. The other ring delivers polarized electron or positron beam with an en- ergy of 27.6GeV starting with up to 50mA beam current which drops expo- nentially with a life time around 13 hours. These unique features of HERA enables colliding and fixed target ex- periments. Four detectors were installed in HERA. In the north and south the colliding beam experiments H1 (north) and Zeus (south) are installed. These experiments running from 1992 until the end of HERA in 2007 in- vestigated unpolarized nucleon structure functions with collisions of protons and electrons or positrons aiming discovery of lepto-quarks. The experiment HERA-B located in the west used from 2000 to 2003 only the accelerated protons on a fixed target to examine charm and bottom meson production. The HERMES experiment in the east was running only with accelerated electrons or positrons on polarized or unpolarized gaseous target, designed to contribute in solving the nucleon spin puzzle initiated by EMC experiment [Ash+88], [Ash+89]. Figure 3.3: HERA consist on two concentric storage rings. The green circle shows the proton ring while the blue circle show the lepton beam which is self polarized due to the Sokolov-Ternov effect [Ter+61], [Sok+63], [Sok+64]. Electron/Positron-Beam HERA consists of two beams as shown in figure 3.3. The one beam of protons rotates counter clockwise while the other beam of leptons rotates clockwise. CHAPTER 3. THE HERMES EXPERIMENT 29 The proton beam as well as the e± beam are not continuously filled beams. The whole HERA is segmented into 220 numbered buckets to hold the particles if desired. As the time slice for each bucket is 96ns the rotation frequency of a particle is around (220 ∗ 96ns)−1 ≈ 47kHz. Each bucket which is filled with accelerated particles is called bunch. The setup of buckets and bunches has a characteristic shape starting at Bunch 1 and is segmented into 3+1 parts. There are three so called trains of bunches and a bunch free zone. Almost all buckets of these trains are filled with particles and are colliding bunches. Exceptional some few buckets called pilot bunches are only filled with one type of particles in order to monitor and tune the corresponding beam as they do not collide with the other beam. In the bunch free zone none of the buckets are filled. This give the possibility to imprint reference signals inside the detectors for monitoring and tuning like for gain monitoring systems without interference of background generated by accelerated particles. Figure 3.4: Self polarization of the lepton beam due to an asymmetry in synchrotron radiation described by Sokolov-Ternov effect and intentional de- polarization at the end [Bar+93]. The proton beam is unpolarized. Due to acceleration in the arcs of high- energy storage rings the helicity of leptons can flip by emission of synchrotron radiation described by the Sokolov-Ternov effect [Ter+61], [Sok+63], [Sok+64]. Due to a small asymmetry in the spin-flip amplitudes the transverse state is preferred which lead to a self polarization of the lepton beam. With the help of spin rotator magnets the helicity can be changed to longitudinal CHAPTER 3. THE HERMES EXPERIMENT 30 orientation and vice versa[Buo+86]. Before the upgrade of HERA in 2001 only the HERMES experiment used the longitudinal polarized beam. The accumulated beam spin orien- tation was in the most time in the transverse state in order to improve the beam polarization. The time independent polarization of the leptons can be described by N+ − N− (3.1) PB = N+ + N− where N− and N+ denote the numbers of the two spin states while the time dependent polarization shown in figure 3.4 increases exponential t (3.2) PB (t) = PB 1 − e− τ . The theoretical limit of 92 % polarization can not be reached due to these spin rotation and imperfect alignments of the beam line magnets. Instead a typical polarization of around 60 % after 45 minutes is reached. During the shutdown in 2001 several other spin rotator magnets were installed to provide a longitudinally polarized lepton beam for the collider experiments as well so that the time spending in transverse instead of longi- tudinal orientation decreased. Therefore the rise time grew slightly and the final average polarization dropped a few percentage. In order to quantify the polarizations, two polarimeters as shown in figure 3.3 were installed in HERA using Compton backscattering processes. While the location for the transversal polarimeter [Bar+93] is behind the HERA-B experiment in the west, the location of the longitudinal polarimeter [Bec+02] is downstream after the HERMES experiment in the east. By this setup the disturbance to the experiments by monitoring and measuring of the polarization with the polarimeter is minimized and give possibility to recover the beam condition after these impacts. A detailed introduction into the polarimeters can be found in [Ruh99], [Men01]. 3.2 Target system At HERA four experiments have to coexist. This is limiting the degree of freedom for the setup of the target of the HERMES experiment as the life time of the lepton beam for solid materials, as a target, drops too fast. Therefore HERMES is using a gas as a quasi fixed target [Ste+03], [Bau+03b], [Sch+98]. A novel technique of a target storage cell [Due+92] with an active length of 40cm has been developed to be used for the ex- periment. As the beam current roughly follows the shape of an exponential decay CHAPTER 3. THE HERMES EXPERIMENT 31 (3.3) Ilepton (t) ≈ Ilepton (0) exp−t/τ where τ is the beam life time. If τB denotes the beam life time without gas and τG the beam life time with gas the density of the gas is selected to match the beam life time contribution of 45 hours under normal conditions. In this case the following equation holds 1 1 1 (3.4) = + . τG τB 45h This is the regular running condition. At the end of a fill the density is increased to run one further hour. This high density run increases the rate of data taking by more than one order. Due to the limited life time contribution the target areal density is limited to the order up to 1017 protons per cm2 in contrast to around 1024 protons per cm2 for solid materials. This technique gives the possibility to change the type of gas at any time. The experiment can be run with polarized or unpolarized gas target. The lighter gases like 1 H , 2 H or 3 He were polarized while the heavier gases like 4 He, N , N e, Kr, Xe were unpolarized. An illustration of the target system implemented at HERMES is shown in figure 3.5. In the ABS (Atomic Beam Source) [Nas+03] of the target system a dissociator is used to disjoin molecular Hydrogen or Deuteron gas by radio frequencies to generate an atomic beam gas. The polarization is selected in a set of sextupole magnet system due the Stern-Gerlach-effect before it enters the storage cell in order to interact with the lepton beam. The remaining gas is analyzed to determine the properties of the gas for further analysis In the TGA (Target Gas Analyzer) [Bau+03a] the amount of recombined molecular gas is measured while in the BRP (Breit-Rabi- Polarimeter) [Bau+02] the polarization is measured. A usual transversely target polarization of 78% for hydrogen was usable [Air+05b] and routinely a longitudinal polarization of 97% for hydrogen and 92% for deuterium was available [Nas+03]. As the space in the target area was limited by the apparatus to pro- vide polarized and unpolarized gas targets the recoiling particles were not detected. The full kinematic are computed by the missing invariant mass technique and the kinematic resolutions are therefore limited by the resolu- tion of the forward spectrometer. 3.3 Forward spectrometer As mentioned before the HERMES forward spectrometer [Ack+98] (figure 3.6) was originally designed to measure the nucleon spin budget. For studies CHAPTER 3. THE HERMES EXPERIMENT 32 Figure 3.5: Equipment of the polarized gas target [Bau+03b]. The target gas is dissociated and polarized in the ABS (Atomic Beam Source) [Nas+03] be- fore entering the storage cell. The TGA (Target Gas Analyzer) [Bau+03a] and BRP (Breit-Rabi-Polarimeter) [Bau+02] are used to analyze the re- maining gas. A set of sextupols magnets with strong, medium and weak field transitions are used. on absolute cross sections or different years of data taking the knowledge about the luminosity is of great importance which is measured with a lumi- nosity monitor. For the reconstruction of interactions with the accompanied particles a system of detectors is established for track reconstruction and for particle identification. For both system a short introduction will be given. The HERMES forward spectrometer consists of two parts, where the up- per and the lower part are symmetric. The configuration of the HERMES detector was modified several times for improvements of particle identifica- ˆ tion and for extensions of the physics program. After 1997 the Cherenkov ˇ Counter was replaced by the RICH (Ring Imaging Cherenkov), the damaged VC was removed and DVC was installed. In 2000 the LW (Lambda Wheels) were installed to extend the HERMES acceptance and in 2005 the RD (Re- coil Detector) was installed to detect the properties of recoiling particles. 3.3.1 Luminosity monitor In order to compare and understand the results of physical analysis the knowledge of the absolute luminosity in DIS studies is very important. Tar- get states and beam states have to be combined in order to calculate cross section asymmetries with relative luminosities. Therefore a luminosity mon- CHAPTER 3. THE HERMES EXPERIMENT 33 Figure 3.6: An illustration of the HERMES forward spectrometer. The lepton beam enters from the left and interacts with the gas target in the target cell (yellow). The tracking system consisting of DVC, FC, MC and BC which are colored in red. The RICH, TRD, hodoscope preshower detector and calorimeter for the particle identification are in green. The lepton beam is shielded by a septum steel plate. The Luminosity monitor for absolute normalization is shown in light blue. The acceptance of the spectrometer is indicated by dashed lines. itor was installed [Ben+01], [Ben98]. (a) (b) Figure 3.7: Luminosity monitor [Ben+01]. (a) The setup of the luminosity monitor for the absolute normalization. (b) Front view of the distributions of reconstructed trajectory of elastic scattered particles. In case of a positron beam Bhabha scattering e+ e− → e+ e− and pair annihilation e+ e− → γγ are used to measure the luminosity while for the case of electron beam Møller scattering e− e− → e− e− is investigated. CHAPTER 3. THE HERMES EXPERIMENT 34 In both cases the event rate R of these very well known QED processes is corrected for normalization, acceptance and efficiency to determine the absolute luminosity L [Ben+01] by R (3.5) L= ∆Ω Ωǫ (dσ/dΩ) where R is the event rate of these processes, ǫ denotes the efficiency and integrated over the acceptance space angle ∆Ω. Figure 3.8: Scatterplot of energy depositions in the right side versus left side. The dashed line separates Bhabha events from background [Ack+98]. The setup of the luminosity monitor located between the electromagnetic calorimeter is shown in figure 3.7. It consists of a pair of electromagnetic calorimeters left and right of the beam pipe which are moved close to the beam after the data taking was declared to reduce possible radiation damage. ˇ Both are made of a 3x4 matrix of N aBi(W 04 )2 (NBW) Cherenkov crys- tals each with a size of 22 × 22mm 2 and a length of 200mm. The readout is made by photomultiplier tubes of type Hamamatsu R4125Q. A typical response on elastic scattering is shown in figure 3.8. 3.3.2 Magnet system The spectrometer magnet consist of two magnets. The main magnet is of the horizontal type with a maximum deflecting power of Bdl = 1.5T m for CHAPTER 3. THE HERMES EXPERIMENT 35 the momentum reconstruction while the second magnet is a correction coil with an deflecting power of 0.08T m. The magnet field was modeled and calculated with MAFIA and TOSCA simulation programs and agreed within a few percent with measurements with 3D-Hall probes with an automated 3D-mapping machine. The overall reproducibility of the results are below a permille. This fine 3D-map of the magnet field is implemented into the track reconstruction algorithm as a lookup table. The main magnet has field clamps in front and behind in order to mini- mize the fringe field at the position of the drift chambers which are part of the tracking system. For safety and for reduction of power consumption it was running at 1.3T m. With this setup the remaining fringe fields at these positions are below 0.1T. A secondary effect is to protect sub-detectors be- hind the magnet from synchrotron radiation produced in HERA. The space in the magnet center leaves an acceptance of ± (40 − 140) mrad in vertical direction and ±170mrad in the horizontal direction. Even if the lepton beam is shielded with an 11cm thick steel plate the main magnet has some effects on the lepton beam. The effect to the proton beam is neglectable due proton beam energy. Therefore a second magnet was installed inside the shielding of the lepton beam pipe. The main two functions of these correction coils are to reduce the disturbance generated by the main magnet and to compensate the transverse component of the lepton beam. 3.3.3 Tracking system The main task of the tracking system is to reconstruct the tracks and there- fore particle momentum as illustrated in figure 3.9. Additionally the corre- sponding hits in the detectors for particle identification can be correlated. Furthermore the kinematics due to the deflection by the magnet can be determined. The geometrical resolution of a fully operational tracking system is lim- ited by Bremsstrahlung while passing the wall of the target cell for electrons, of the stainless steel vacuum window and of the first tracking detectors. The tracking system is shown in the figure 3.6 in red color. It consists originally in downstream direction of VC (Vertex Chambers), DVC (Drift Vertex Cham- bers), FC (Front Chambers), MC (Magnet Chambers) [And+01] and BC (Back Chambers) [Ber+95], [Ber+98]. In 1998 the damaged VC was re- moved and in 2000 the LW (Lambda Wheels) were installed to increase the acceptance of the HERMES forward spectrometer. CHAPTER 3. THE HERMES EXPERIMENT 36 VC The VC 3.6 is a micro strip gas chamber for high precision reconstruction downstream right after the target cell area and is therefore very close to the beam pipe. The stripe width of 7µm and a distance of 193µm yields in a very narrow resolution of around 65µm. Due to an accident during beam dump the VC was seriously damaged and therefore removed in 1998 in order to minimize further disturbance. Figure 3.9: Schematic illustration of the tracking system. Long tracks reach- ing all tracking detectors, while short tracks are deflected by the HERMES magnet out of the acceptance of the BC. Drift chambers and proportional chambers In order to simplify the handling all the other detectors for tracking consist of drift chambers built as a sandwich of three planes with vertical or ±300 tilted wires. The DVC (Drift Vertex Chambers) and FC (Front Chambers) [Bra+01] in front of the magnet is used to reconstruct the front part of the tracks, while the BC (Back Chambers) [Ber+95], [Ber+98] behind the magnet are used to measure the back part of the tracks. By combining these information the momentum of the charged particles can be measured due to deflection in magnet field. The chambers has for simplicity the same setup and running with the same gas mixture of Ar (90%) CO2 (5%) CF4 (5%) which provides very high CHAPTER 3. THE HERMES EXPERIMENT 37 drift velocity and is non-flammable for security. Even this tracking are very accurate to determine the momentum the MC (Magnet Chambers) [And+01] is of great help to resolve ambiguities of track finding and to measure low momentum particles which does not reach the back part of the spectrometer which are called short tracks. The MC which has to operate in strong magnet field has slightly different setup. It is built as MWPC (Multi Wire Proportional Chamber) with a optimized gas mixture of Ar (65%) CO2 (30%) CF4 (5%). For the readout of the detectors ASP (Amplifier-Shaper-Discriminators are used to digitize by Fastbus Multihit TDC with a time accuracy of 0.5ns. With the usual wire diameter of around 25µm and at a distance of few mm results in a spatial resolution of around 300µm per plane. The resolution of the momentum reconstruction was between 0.7% to 1.25% before 1998 and was since then between 1.5% to 2.5% due to dismount of the precise but damaged VC and as furthermore it was decided to put an additional material (aerogel) for the new RICH into the track path. These modifications were made to enhance the physics program of the HERMES detector. LW Last upgrade of the tracking system was the LW (Lambda Wheels) detector based on silicon wafer and located behind the target cell in order to extend the acceptance of the forward spectrometer and to detect λ-hyperons in wide kinematic domain. Detailed informations about the setup can be found in [Hee03] and recent results are documented in [Dem07], [Rei07]. Track reconstruction Due to the simple setup of the tracking system the track reconstruction can be done very efficiently by a tree search for the track finding and by a fast momentum determination with a look-up table for the track kinematics [Ack+98]. The tree search algorithm as illustrated in figure 3.10 works very fast and simple. The possible window for the track pattern are bisectioned in each step. As the tracking detector has a size of around three meters and a resolution of around 250µm after around 14 iterations the binary tree search determinate independently for the front part and the back part of the HERMES spectrometer into partial tracks. These front partial tracks are combined within specific spatial tolerance to find back partial tracks which have a starting vertex near the interaction region and reaching the other detector of the particle identification system. After the track finding the momentum reconstruction is done by combin- ing tracking information in front and behind the magnet as charged particles are deflected while passing through magnetic fields. Under normal condi- CHAPTER 3. THE HERMES EXPERIMENT 38 tions the reconstruction in an inhomogeneous magnet field requires usually a high amount of calculations. The HERMES track reconstruction uses a simple look-up table to re- construct the momentum which contains all relevant informations of space points before and after the magnet with a relative momentum resolution contribution of less than δp/p ∼ 0.5%. The table has around 0.5M en- tries. This momentum reconstruction algorithm is therefore very efficient and require only low performant computer hardware. Alignment The resolution of the tracking system depends essential on a proper align- ment of the sub detectors. In order to align the tracking system dedicated runs were taken without a magnet field to gain straight tracks to measure the relative position of the detectors. Due to the simple layout of the cham- bers a simple tree backtracking algorithm can be applied to determine the relative offsets. Additionally a laser tracking system [Shi+98] is used to monitor online relative movements of the detector. Due to the ramping of the magnet unpredictable movements of the detectors can been detected and corrected in situ. The Ks meson decay can be used to check the absolute calibration of the spectrometer and the performance of the tracking system. The result shown in figure 3.11 gives a very good agreement with the PDG (Particle Data Group) [PDG08] on one part of permille accuracy. 3.3.4 Particle identification The major interaction of particles passing through material is ionization, ˇ Bremsstrahlung, pair production or Cherenkov radiation. The amount of energy deposition or deflection of charged particles depend on the mass, charge and momentum of the passing particle and therefore particle identi- fication is possible. The system for particle identification, one of the key features of the HERMES detector, consist of four detectors. Due to this complementary combination the HERMES detector has a very good particle separation. ˇ These detectors are RICH (Ring Imaging Cherenkov) [Asc+00], [Ako+02], TRD (Transition Radiation Detector) [Eme96],[Thi96], hodoscope preshower detector and an electromagnetic calorimeter [Ava+98]. RICH The RICH [Ako+02] illustrated in figure 3.12 replaced the threshold Ring ˇ Imaging Cherenkov counter in 1998 and allows to provide very efficient sep- ˇ aration of pions, kaons and protons by Cherenkov radiation in a wide mo- CHAPTER 3. THE HERMES EXPERIMENT 39 Figure 3.10: Illustration of the HERMES track finding algorithm [Ack+98] by bisectioning tree search in each iteration step. The resolution is step wise doubled to determine the trajectories of the tracks. CHAPTER 3. THE HERMES EXPERIMENT 40 Figure 3.11: The invariant mass of π + π − pairs [Ack+98]. The re- constructed KS mass (497.4M eV ) agrees very well with the PDG value (497.7M eV ). Figure 3.12: The setup of the upper half of the RICH (Ring Imaging ˇ CHerenkov) detector [Ako+02] which is one of the key part of the particle identification. The bottom half has similar construction. mentum range. The characteristic angle θC of the light cone is correlated with the speed of the particle thus with the momentum by 1 (3.6) θC = nβ where n denotes the refractive index of the material while β = v/c with v CHAPTER 3. THE HERMES EXPERIMENT 41 ˇ Figure 3.13: The angle of the Cherenkov cone versus the momentum for the major particle types are shown. On the left side the Monte Carlo simulation is shown as well as on the right side real data are included [Haa07]. as speed of particle and c as speed of light. As shown in figure 3.13 the RICH can contribute to separate leptons and hadrons for momentums below 4GeV very well. As the lepton energy spectrum for analysis of exclusive events has it's main contribution above 5GeV the RICH is not taken into account for this work. A very detailed description of the RICH detector can be found in [Asc+00] and [Ako+02]. TRD Charged particles passing different dielectric mediums produce radiation on the transition in order to match the different Coulomb field in the material. The mean energy E of the radiation in case of ultra-relativistic condition can be expressed as 2 (3.7) E = αem ωP γ 3 where αem = 1/137 is the electromagnetic fine structure constant, γ is the Lorentz factor and ωP is the plasma frequency. The transition radiation is emitted in a cone around the particle trajec- tory with an opening angle of θ = 1/γ. As the Lorentz factor γ for leptons are much higher due to the larger mass than for hadrons this allows a lepton hadron separation by energy deposition. As the probability for a transition radiation is very small a large amount of transitions of material is required. This can be done by a sandwich design as illustrated in figure 3.14. CHAPTER 3. THE HERMES EXPERIMENT 42 Figure 3.14: The upper half of the TRD [Ack+98] setup is illustrated with a lepton and pion trajectory. The opening angle of the lepton path is due to better visibility not to scale. The TRD setup consists of six independent modules with an active area of 72.4 × 325cm2 . Each modules has a sequence of radiator and PWC (Pro- portional Wire Chamber) in order to provide informations about energy deposition and location. The radiator with a thickness of 6.35cm is built by fibers with 17µm to 20µm diameter and a density of 0.059g/cm3 . This corresponds to an average of 267 dielectric layers. The PWC has a conven- tional design with 256 vertical wires of 75µm gold-coated Be-Cu-compound material separated by 1.27cm. For an easy in situ replacement the wires are crimped and are positioned with an accuracy of 25µm. The chambers has a thickness of 2.54cm and was filled with Xe (90%) CH4 (10%) detector gas for highly efficient X-ray absorption. The analog signals are amplified and transmitted as differential signal with twisted-pair flat cables to the Fastbus ADC for readout. The response for a single TRD module and for a truncated mean of six TRD modules are shown in figure 3.15. Electromagnetic calorimeter The electromagnetic calorimeter shown in figure 3.16 has two functions to provide a first-level trigger for scattered beam leptons and to provide particle identification for the off-line analysis. It consists of radiation resistant lead-glass called F101. Each of the upper and lower walls was built with 42 times 10 blocks with a active area of 9x9cm2 each. The length of such a block is 50cm which is about 18 radiation lengths. In order to minimize optical cross talks between the blocks they CHAPTER 3. THE HERMES EXPERIMENT 43 (a) (b) Figure 3.15: Response of TRD [Ack+98] for leptons and hadrons as count versus energy deposition, integrated over all momentums. (a) Response for a single TRD module. (b) Response for truncated mean of six TRD modules. Figure 3.16: Illustration of the electromagnetic calorimeter together with the hodoscope preshower detector H2 in front [Ack+98]. are covered on the side with 51µm thick aluminium mylar foil and with 0.127mm thick tedlar foil. Each block has a PMT (Photomultiplier tube) of type Philips XP3461 on the downstream side. In order to prevent radiation damage both walls are moved in a distance of 50cm to the beam during injection. CHAPTER 3. THE HERMES EXPERIMENT 44 The performance of the calibration of the electromagnetic calorimeter is shown in figure 3.17. The energy E is measured by the electromagnetic calorimeter while the momentum p is reconstructed by the tracking system of the forward spectrometer. The ratio r = E/p ≈ 1.00 ± 0.01 demonstrate the ≈ 1% uniformity of the response. The long term stability is of the same order and includes the effect of radiation damage over the years as well. In order to check the overall calibration the reconstruction of the π 0 decay is used. The reconstructed π 0 mass from 2γ events are shown in figure 3.17 and is in good agreement with the PDG (Particle Data Group) value. A more detailed description of the electromagnetic calorimeter can be found in [Ava+98]. (a) (b) Figure 3.17: Performance of the electromagnetic calorimeter [Ack+98]. E are the measured energy while p is the reconstructed momentum. (a) Distribution of E/p with center value 1.00 and with width 0.01. (b) Dis- tribution of π 0 mass reconstructed from 2γ cluster event. The fit with a gaussian distribution yields to the center (134 ± 0.2) M eV and to the sigma (12.5 ± 0.2) M eV and is in good agreement with the PDG value. The hodoscope for trigger and PID The hodoscope detectors H0, H1, H2 consist of fast plastic scintillators. They are mainly employed as trigger detectors. On the other hand they contribute to particle identification by measuring energy deposition as well. The setup of the lower and upper hodoscopes are identical. The location of the H0 is in front of the HERMES magnet and has a active area of 60x20cm2 and a thickness of 3.2mm as illustrated in figure 3.18. Each H0 is readout by two PMT of type Thorn EMI 9954SB. CHAPTER 3. THE HERMES EXPERIMENT 45 The hodoscope H1 is before and the hodoscope H2 is behind the TRD as shown in figure 3.6. Each hodoscope has the same setup as shown in figure 3.16 and consist of 84 vertical scintillator modules which are 1cm thick and has an active area of 9.3x91cm2 of a fast scintillator material with large attenuation length of 300cm to 400cm. The modules have an overlap region of around 2cm to 3cm to maximize efficiency. The hodoscope H2 has additional a passive radiator consisting on 11mm (2 radiation lengths) of Pb with 1.3mm stainless steel in front. Each of the scintillating modules are readout with a PMT of type Thorn EMI 9954SB and provide together with H0 fast signals for first level trigger. With this setup a striking lepton produces a broad distribution of de- posited energy with a mean of 20M eV to 40M eV which has a weak depen- dency on the energy of the passing lepton. In comparison to around 2M eV by a pion this gives the opportunity to separate hadrons from leptons clearly as shown in figure 3.18. (a) (b) Figure 3.18: (a) Technical drawing of the hodoscope H0 which is mainly used for the first level trigger. (b) Response of hodoscope preshower detector H2 which utilize particle identification by energy deposition. PID performance The performance of the particle identification system in shown in figure 3.19 where the PID value represents the accumulated informations of the contributing detectors about the particle. A very detailed description of the PID value which is a likelihood estimation can be found in [Kai+97], [Wen99], [Wen01]. CHAPTER 3. THE HERMES EXPERIMENT 46 Figure 3.19: The performance of the PID system [Ack+98] shows a clear separation between leptons and hadrons. The leptons are located left while the hadrons are on the right side. Chapter 4 The Recoil Detector To improve the exclusivity and the resolution of the kinematic variables the HERMES collaboration decided to upgrade the HERMES Detector with the RD (Recoil Detector)[Kai+02], [Sei04] to detect the recoiling particles directly. To install the RD the equipment for the polarized gas target had to be removed as illustrated in figure 4.1. During the maintenance shutdown in winter 2005 the Recoil Detector was installed in front region of the HERMES detector and data taking was running until the HERA shutdown in June 2007. The readout implementation of the SFT (Scintillating Fiber Tracker) which is one of the key part of the RD and the readout integration of the RD into the HERMES data acquisition are the main part of this work. (a) (b) Figure 4.1: The HERMES Detector front region before (a) and after (b) the installation of the Recoil Detector as 3D illustration gives a good impression about the space limitation. 47 CHAPTER 4. THE RECOIL DETECTOR 48 4.1 Design requirements and realization The resolution of the DVCS kinematics and the suppression of background is limited in the HERMES forward spectrometer by the electromagnetic calorimeter and the impossibility of the detecting the recoiling particle. The aim of the Recoil Detector is therefore to enhance the event selection to improve the kinematic resolution for physical analysis and to be able to make the events more exclusive. This is done by detecting and reconstructing the properties of the recoiling particles directly. Furthermore the event selection is improved by suppression of background. Extensive Monte Carlo studies on reactions of the type (4.1) eN → e′ γX with e as incoming lepton, e′ as scattered lepton, γ as the produced real photon, N as target nucleon and X as rest of products were carried out in order to determine selections for exclusive reactions. The remaining in- 2 variant mass Mx of the squared four-momentum properties of the known particles can be expressed as (4.2) 2 Mx = p2 + p2 − p2′ − p2 e N e γ where pe and pe′ are the four momentum of the incoming and scattered lepton, pN denotes the four momentum of the off scattered nucleon target and pγ is the four momentum of the produced real photon. Figure 4.2: Missing invariant mass distribution for different reactions. The vertical lines constraints the event selection to −2.25 ≤ (Mx /GeV )2 ≤ +2.89. CHAPTER 4. THE RECOIL DETECTOR 49 The contribution of different types of reactions are shown in figure 4.2. For the exclusive event selection the used range of the missing invariant mass is (4.3) −2.25 ≤ (Mx /GeV )2 ≤ +2.89. The contribution by associated Bethe-Heitler process also called ∆-reso- nance is in this case around 11%, while the contamination by semi-inclusive processes are around 5%. These Monte Carlo studies [Kra05] indicate the requirement to detect recoiling protons in the momentum range between 50M eV up to 1400M eV which are illustrated in figure 4.3. The dependence particle momentum p versus the polar angle θ for several alternative reactions are shown. Addi- tional properties of the Recoil Detector are summarized in table 4.1. Parameter Unit Ideal Expected Performance Performance p-acceptance M eV /c 50-1400 106-450 (SSD) 250-1400 (SFT) 600-1400 (PD) θ-acceptance rad 0.1-1.35 0.4-1.35 (SSD) 0.7-1.35 (SFT) 0.78-1.90 (PD) φ-acceptance rad 2π 4.8 (SSD) > 4.8 (SFT) > 4.8 (PD) p-resolution 1 < 10% 3-9% for p < 500M eV /c 5% 13% for p > 500M ev/c t-resolution GeV 2 < 0.07 0.01-0.07 for t < 0.03 0.07 − 0.2 for 0.3 < t < 10 φ-resolution rad < 0.05 0.031 (SSD) 0.008 (SFT) 0.1 (PD) π/p PID range M eV /c 50-800 135-650 π+ rejection factor 1 > 10 > 10 for p < 650M eV /c ∆ suppression 1 > 90% 92% Table 4.1: Summary of the properties of the Recoil Detector and for the sub detectors if available [Kai+02], [Hoe06]. By combining a Silicon Strip Detector, a Scintillating Fiber Tracker and a Photon Detector inside of a 1T magnetic field the momentum of the de- tected particles can be reconstructed and allowing particle identification. A schematic overview of the Recoil detector is shown in figure 4.4. CHAPTER 4. THE RECOIL DETECTOR 50 Figure 4.3: Distributions of particle momentums p versus polar angle θ studied with Monte Carlo simulations for (a) DVCS process, (b) DVCS / BH interference term, (c) ρ-meson production and (d) ∆ resonances or as- sociated BH-process [Vil08]. The drawback for these improvements of the HERMES forward spec- trometer is the removal of the polarized gas target due to limited space in the front region of the HERMES detector. The remaining data taking by the HERMES experiment until the shutdown of HERA in 2007 was done therefore with unpolarized gas of hydrogen and deuterium. The installation of the Recoil Detector into the HERMES detector is shown in figure 4.5. To take the new spatial dimensions of the RD and the different target gas system into account a new target cell was built. The Recoil detector is a collaborative effort with contributions by • Infrastructure by CHAPTER 4. THE RECOIL DETECTOR 51 Figure 4.4: Overview of the Recoil Detector. Figure 4.5: HERMES forward spectrometer upgraded with the Recoil De- tector. DESY Hamburg / Germany • TC (Target Cell) by INFN Ferrara / Italy • SSD (Silicon Strip Detector) by CHAPTER 4. THE RECOIL DETECTOR 52 DESY Zeuthen / Germany University of Erlangen / Germany University of Glasgow / Scotland • SFT (Scintillating Fiber Tracker) by University of Giessen / Germany • PD (Photon Detector) by LNF Frascati / Italy University of Gent / Belgium • SCM (SuperConducting Magnet) by PNPI St. Petersburg / Russia 4.2 Target cell The target cell have to be adjusted to face the new dimensions of the RD but the major shape remains. The new target cell was produced like the previous ones by the target group [Air+05b] is shown in figure 4.6. As described before the effective length of the target cell was before 40cm. This value is now 30cm which matches roughly the size of the active part of the SSD (Silicon Strip Detector) which stay in the HERA beam pipe vacuum. The thickness of the target cell made by thin aluminum foil was originally 50µm. An accidental damage of the thin foil due to thermal and mechanical stress caused indirect serious damage in the SSD. A new target cell was built with 75µm thickness and installed during the last maintenance shutdown of HERA in June 2006. Several collimators in front of the Recoil Detector protects the target cell and the Silicon Strip Detector from synchrotron radiation. The scattering chamber covering the active area of the Recoil Detector and enclosing the vacuum region are 1.2mm thick and is the other reason for the lower limit of the momentum of detected protons. The previous cooling for the polarized gas target was running with liquid helium in order to improve the target polarization and for a precisely and stabled polarized gas. Running the experiment with unpolarized gas the need for this expensive and difficult cooling with liquid helium is obsolete. This simplifies the construction and servicing of the target cell. While run- ning the gas target around room temperature the cooling can be done with water and the thermal stress can be significantly reduced. Beside this the control of the temperature is better as well. CHAPTER 4. THE RECOIL DETECTOR 53 Figure 4.6: The new adjusted target cell which fits the requirements of the Recoil Detector. 4.3 Silicon Strip Detector Due to the design requirements studied with Monte Carlo simulations the material budget in front of a detector has to be minimized In order to de- tect very low momentum particles. For this the SSD (Silicon Strip Detector) [Pic08], [Vil08] is mounted inside the HERA beam vacuum with a pressure of around 10−9 mbar and is one of the key parts of the RD. This way of imple- mentation yields to measure protons with momentums as low as 50M eV /c. As shown in figure 4.7 the response of silicon for passing protons de- CHAPTER 4. THE RECOIL DETECTOR 54 (a) (b) Figure 4.7: Response of a 300µm silicon layer on proton penetration. (a) Bethe Bloch formula [PDG08], [Vil08] gives the energy deposition response versus incident proton momentum. The two vertical lines illustrate the de- signed active range of the silicon modules for the SSD. (b) The arrows indi- cate increasing proton momentum [Haa07]. Protons with momentum range 40M eV /c to 106M eV /c mostly stuck in the first layer. Protons with mo- mentum range 106M eV /c to 135M eV /c mostly stuck in the second layer. Protons with momentum above 135M eV /c passes both layers. scribed by the Bethe-Bloch formula is very simple. The momentum of the recoiling proton can be reconstructed by energy deposition due to ioniza- tion. Combining two layers of silicon improves the momentum resolution. The typical triangle shape of response can be separated in three parts. The first part is due to stopped particle with very low momentum from 40M eV /c to 106M eV /c in the first layer without a response in the second layer. In the second part the particle with momentum range of 106M eV /c to 135M eV /c passes the first layer but stuck in the second layer. In the third part particles in the momentum range from 135M eV /c passes both silicon layers. The setup of the SSD consists of eight SSD modules as illustrated in figure 4.8 which are built in a diamond shape with an inner and an outer layer. Each SSD modules has two double sided silicon wafers called TIGRE sensors with parallel or perpendicular orientation of the 128 strips on each side. Each of the silicon wafer with thickness of around 300µm has an active area of 10 × 10mm2 . Each strip has a pitch width of 758µm and a distance of 56µm to the neighboring strips. This provides two space points per track for track reconstruction with a resolution of around 222µm and particle identification for the momentum range from 106M eV /c up to 500M eV /c. As the wafers are a few cm away CHAPTER 4. THE RECOIL DETECTOR 55 from the beam the polar angular acceptance are between 0.1rad and 1.35rad. Due to the ceramic frame which holds the silicon sensors the azimuthal angular acceptance is around ∼ 70%. (a) (b) Figure 4.8: Setup of the Silicon Strip Detector. (a) The drawing illustrates the diamond shape surrounding the target cell in the HERA beam vacuum of the scattering chamber. (b) Photo shows one of the eight silicon modules. In order to extend the dynamic range [Hri+05] each strip has a charge division circuit with two outputs to the ADC (Analog to Digital Converter) as shown in figure 4.9. The high gain channels are used if the signals are not in the saturation range. In case of saturation in the high gain channel the low gain channel are used to measure the energy deposition. The dynamic range of the energy deposition are up to 7M eV . The ADC consists of HELIX128 chips which was used for the LW (Lambda-Wheel) detector [Hee03]. 4.4 Scintillating Fiber Tracker One of the major components of the Recoil Detector is the SFT (Scintillat- ing Fiber Tracker) [Hoe06]. The aim of this detector is to provide several space points to reconstruct the momentums of charged particles due to the deflection while passing the magnetic field. Furthermore the SFT contribute to the particle identification with momentum below 650M eV /c. In combina- tion with the PD (Photon Detector) particle identification can be provided for particles with higher momentum. 4.4.1 Detector Particles passing material create electromagnetic radiation. In figure 4.10 this response for different materials and different particles are shown. The range indicated by red vertical lines can be used to identify passing protons CHAPTER 4. THE RECOIL DETECTOR 56 Figure 4.9: The dynamic range of the silicon sensors are extended by factor 5 by charge division circuits as illustrated on the bottom. The ADC value of the high gain channel is used to reflect the accumulated input charge for the non-saturation regime. In case of saturation of the high gain channel the value of the low gain channel is used instead. [Hri+05] by the amount of deposited energy. Pions can be used as minimum ionizing particle as indicated by the green lines. Figure 4.11 illustrate sample response of protons with different momen- tum and pions. Due to multiple scattering inside the fibers the response is smeared slightly to wider distributions. The setup of the SFT consists of two barrels with two layers as indicated in figure 4.12(a) in order to support the track reconstruction. The fibers in the inner layer of the barrels are parallel to the beam axis while the outer layers of the barrels have an angle of 10o to the beam axis. In figure 4.12(b) the internal structure of a SFT module is illustrated. The overlap ensures a complete azimuthal coverage by the SFT detector. Due to this setup the SFT provides up to two additional space points. Together with the space points provided by the other subdetectors the particle tracks can be completely reconstructed. The complete SFT detector is shown in figure 4.13. The active length CHAPTER 4. THE RECOIL DETECTOR 57 10 8 6 − dE/dx (MeV g−1cm2) H2 liquid 5 4 He gas 3 C Al Fe 2 Sn Pb 1 0.1 1.0 10 100 1000 10 000 βγ = p/Mc 0.1 1.0 10 100 1000 Muon momentum (GeV/c) 0.1 1.0 10 100 1000 Pion momentum (GeV/c) 0.1 1.0 10 100 1000 10 000 Proton momentum (GeV/c) Figure 4.10: Mean Energy loss versus momentum for different charged particles in different material. The response of scintillating fibers are similar to carbon. The vertical lines indicates the operational ranges for protons (red) and pions (green). [Eid+04] counts [arb. units] 1600 pions protons, 900 MeV/c 1400 protons, 600 MeV/c protons, 450 MeV/c 1200 protons, 300 MeV/c 1000 800 600 400 200 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 deposited energy [MeV] Figure 4.11: Sample energy response distributions of the selected scintillat- ing fibers for pions and protons for different momentums. [Hoe+07] CHAPTER 4. THE RECOIL DETECTOR 58 (a) (b) Figure 4.12: Setup of the SFT (Scintillating Fiber Tracker). (a) Illustration of the concept for track reconstruction. (b) Photo of one stereo SFT module. of the fibers is around 280mm with an inner diameter of around 218mm while the outer diameter is around 375mm. All fibers have 1mm diameter. The scintillating fibers for the detector are from Kuraray company, model SCSF-78M. The light guides between the detector and the photomultiplier tubes are built with clear Pol.Hi.Tec. fibers. The inner barrel consists of 1318 parallel and 1320 stereo fibers while the outer barrel has 2198 parallel and 2180 stereo fibers. This yields to a azimuthal resolution of around 8mrad. As two fibers in the outer barrel are mapped to one MAPMT (Multi Anode Photomultiplier Tube) pixel almost 5k channels have to be readout. Particle identification for a momentum range between 250M eV /c and 650M eV /c is possible. Together with the PD (Photon Detector) particles with higher momentum can be identified. Even the SFT provides full azimuthal angular coverage the azimuthal angular acceptance for full tracking is limited by the SSD supporting frame to ∼ 70%. The polar angular acceptance is between 0.7rad and 1.35rad. CHAPTER 4. THE RECOIL DETECTOR 59 Figure 4.13: Realization of the detector right before installation without. On the right the connector of the PD covered with withe tape is visible. 4.4.2 Detection of light by MAPMT The scintillating light is detected with H7546B MAPMT (Multi Anode Pho- tomultiplier Tube) from Hamamatsu [Ham00]. These head-on type 8x8- MAPMT has the entry on the front side and the connector of the back side. It provides high speed response while the internal cross talk is very low despite of the high electronic density as illustrated in figure 4.14. Each MAPMT pixel has 13 stages while an additional Dynode 12 output signal is available by accumulating all channels of the 12th stage. This Dynode 12 signal can be used for fast signal tracking. The output of each channel of the 64ch-MAPMT works like a signal source. In order to maximize the available signal amplitude a resistive adap- tion to the digitizing units which works like a signal drain is required. In this case a CDC (Charge Division Circuits) illustrated in figure 4.15 consisting of a resistors-capacitors-circuit sitting on a PCB (Printed Circuit Board) is used. In order to avoid malfunction by weak galvanic connection the MAPMT are soldered on to the CDC-PCB as shown in figure 4.16. The square sol- dering bed are surrounded by the resistance adaption. CHAPTER 4. THE RECOIL DETECTOR 60 (a) (b) Figure 4.14: MAPMT Hamamatsu H7546B [Ham00] for detection of scin- tillating light. (a) Photo of the core H5900-00-M64 MAPMT (Multi Anode Photomultiplier Tube) without housing and voltage divider. (b) The layout of the cathode plane illustrate the high density package of the 64ch-MAPMT. 10pF PMT GASSIPLEX 22k 82pF 22k Figure 4.15: Interface between MAPMT as signal source and the track and hold unit Gassiplex as signal drain. CDC (Charge Division Circuit) to adjust resistance of MAPMT and Gassiplex chips. 4.4.3 Analog to Digital Converter To improve the particle identification beside the topology of the track as well as the deposited energy inside the SFT have to be measured as sim- ple hit informations generated by discriminators are not sufficient. Since 5120 MAPMT channels have to be readout and as the amount of space in the experimental area are limited standard hardware did not fit the neces- sary requirements. Therefore the inhouse high density ADC readout of the HADES (High Acceptance Di-Electron Spectrometer) experiment was used as the cost per channel are very low as well. A very detailed documenta- tion about this Analog to Digital Converter can be found in [Kas+99] and CHAPTER 4. THE RECOIL DETECTOR 61 Figure 4.16: Photo of the two PCB of the CDC and PFM. The CDC- PCB hold the soldered MAPMT and the charge division circuits while the PFM-PCB contains the electronics for the digitization and sparsification. [Boe00]. The Gassiplex chips [San+94], [San+01] which was developed at CERN were used as track and hold unit to store the analog signal for queued digi- tization with one fast and precise ADC. In figure 4.17 the internal setup of this chips is shown. All 16 input channels have charge sensitive amplifiers on the first stage. The second stage consist of selectable filter in order to change the polarity of the analog signal if desired. The third stage is a shaper with a integration time of around 600ns. The output of the shaper are stored with a track and hold unit. On the last stage a 16:1 multiplexer is used to make the analog signal accessible to the Gassiplex output. Figure 4.18 illustrate the setup of the ADC-PCB named PFM (Prepro- cessing Frontend Module). The tracked analog signal of the four Gassiplex chips are routed over a 4:1 videomultiplxer El441C [Ela96] from Elantec Inc. to the ADC chip ADS820 [AD08a] from Analog Devices, Inc. (former Burr-Brown Corp.). A threshold sparsification unit is implemented in or- der suppress small signals. All the components are controlled by a FPGA (Field Programmable Gate Array) XC4005E [Xil] from Xilinx which can be programmed in order to reflect improvements in the readout handling. The CHAPTER 4. THE RECOIL DETECTOR 62 CDC-PCB with the MAPMT and the PFM-PCB for the digitization are shown in figure 4.16. Figure 4.17: Sketch of the Gassiplex chips [San+94], [San+01] which was developed at CERN. This chips works as a track and hold unit and has an internal multiplexing to map 16 input channels on one output channel. In figure 4.19 a sample MAPMT signal is shown which has a narrow pulse width of few ns. As mentioned before the Gassiplex has a integration time of around 600ns. The figure 4.20 indicate that the timing of the digitizer trigger is not very crucial. The HERA bunch cycle time is 96ns. This can lead to misinterpretation of the resulting analog signal as several independent interactions can be accumulated. In order to decide if the signal corresponds to the right bunch a secondary readout chain was implemented [Har04]. The selected MAPMT provides with the Dynode 12 output a fast track- ing signal. By shaping the signal with a combination of amplifier and dis- criminator a multi-hit multi-event fast TDC (Time to Digital Converter) provide hit timing information with 100ps resolution for cross check of the corresponding ADC value. 4.4.4 ADC Readout The readout of the SFT based on the hardware of the RICH (Ring Imaging ˇ Cherenkov) readout of the HADES Experiment. Detailed documentation in [Kas+99] and [Boe00] are available to describe the hardware used by the CHAPTER 4. THE RECOIL DETECTOR 63 Figure 4.18: Scheme of the functionality of the PFM-PCB [Kas+99], [Boe00]. The green part is responsible for the digitization. Each of the 64 analog signal from the MAPMT are routed through one of four 16Ch- Gassiplex chips to a 4ch to 1ch video multiplexer. Therefore only one very fast and precise ADC-chips is needed. The gray part contains the Xilinx FPGA as local controller and the memory for the thresholds of the sparsi- fication unit and after sparsification for the remaining data. The blue part represents the buffering of the interfacing signal ADC readout. Therefore the hardware details will be skipped. An general schematic of the readout is illustrated in figure 4.21. The hardware is based on versatile FPGA chips which consists of a grid of logic gates and can be reprogramed for desired functionality. This provides flexible adjustment of the signal flow and of the control flow of the DAQ (Data Acquisition). The external trigger is delivered to the Gassiplex chips of the PFM (Preprocessing Frontend Module). After digitizing of the analog signals the optional sparsification is performed. The readout of the available data can be done either by a direct access to the RC (Readout Controller) via a standard PC or via a DSP (Digital Signal Processor) located on the VME crate controller. As the readout is based on the DTU (Detector Trigger Unit) which controls the readout controller several debugging informations about the internal state were provided to detect malfunction in the readout and to recover within the dead time of the readout. CHAPTER 4. THE RECOIL DETECTOR 64 Figure 4.19: Sample MAPMT [Ham00] signal as input to the track and hold unit. Recoil standalone readout For test runs and maintenance of the Recoil Detector the SFT was readout directly by a PC based DAQ [SFT+06]. For this a simple console based program in the high level C programming language was written. The major informations like counters and event tagging was integrated into the data stream. This gives the opportunity to cross check the data integrity and in case of desynchronization between the ADC and TDC to resynchronize them again. 4.5 Photon Detector The outermost and remaining subdetector of the RD is the PD (Photon Detector). It serves as an electromagnetic calorimeter in order to detect photons in the real experiment coming from decaying neutral particles and to provide a trigger generated by cosmic rays on the cosmic ray test run before the installation of the fully assemble RD into the HERMES experiment for studies on calibration, alignment and for stress tests of the implemented readout of the RD. A short overview about the PD will be given below while a detailed documentation can be found in [Haa07]. As neither the SSD nor the SFT are sensitive for neutral particles the key feature of the PD is to detect photons from decayed particles. A possible source of photons is intermediate ∆+ resonance also called associated BH- process ep → e∆+ which decays in the chain ∆+ → pπ 0 → pγγ. When both produced photons are detected the decaying π 0 can be reconstructed. By CHAPTER 4. THE RECOIL DETECTOR 65 Figure 4.20: Response of the Gassiplex chips for different delay of the underlying trigger system. [Har04] detecting these photons the ∆+ background suppression for hard exclusive processes like DVCS can be improved. Additional feature of the PD is the contribution to the particle identification of protons and pions for momenta above 600M eV /c. The setup of the PD consist of concentric barrels with three alternating layers of tungsten radiator as converter material and scintillating plastic strips for detecting the electromagnetic showers. The complete barrel has an inner diameter of ∼ 190mm, an outer diameter of ∼ 250mm and a active length of ∼ 288mm. The figure 4.22 shows the PD during the construction phase. The figure 4.23 illustrates the construction of the PD in a front and in a side view cut as a schematic while figure 4.24 displays the stripes. The first layer of tungsten in front of the first scintillating layer has a thickness of 6mm while the other two layers of tungsten are 3mm thick. One radiation length corresponds to 3.5mm tungsten. These scintillating layers are build by strips to cover the 2π azimuthal angle and the polar angle between 0.78rad and 1.9rad. The orientation of the strips in the layers with respect to the beam axis are from inner layer to outer layer 0o , CHAPTER 4. THE RECOIL DETECTOR 66 Figure 4.21: Schematic illustration of the SFT readout system. The arriving trigger from the HERMES DAQ is distributed to the Frontend modules to start digitization. Afterwards the available data are read out over the VME based readout controller with conventional PC or with DSP. +45o and −45o . All together 60 strips with a with of 20mm and a thickness of 1mm forming the innermost parallel layer while each of the two stereo layers consist on 44 strips with comparable dimensions. On both sides of each strip a wavelength shifting fiber is embedded. The fibers shifting the generated scintillation light to the best acceptable for PMT wavelength and transporting the shifted light till PMT cathode. These MAPMT H7546B from Hamamatsu are the same as used for the SFT. A shielding is used to attenuate the rest of the magnetic field from outside of the recoil magnet. The electronic signals from the MAPMT are transfered through a transmitter receiver chain from the experimental plat- form to the electronic trailer over more than 30m distance and through additional 80m shielded flat ribbon cables to several CDC (Charge to Dig- ital Converters) V792 from CAEN. These CDC are readout by commercial VME based equipments and standard computer hardware. 4.6 Superconducting Magnet In order to reconstruct the momentum by the deflection of the detected charged particles a superconducting magnet with a field strength of 1T at CHAPTER 4. THE RECOIL DETECTOR 67 Figure 4.22: Internal setup of the Photon Detector during production and before mounting the light tide coverage. The green fibers of the wave-length shifters illustrate the three different orientation (0o ,±45o ) of the strip seg- ments which are covered with white tape. The black fibers are used by the gain monitoring system. The picture is rotated by 900 counterclockwise to present the final orientation. the beam axis is surrounding the RD. The magnet is furthermore protecting the RD, especially the SSD from background electron scattering processes like Møller or Bhabha scattering by bending their tracks to the forward direction. The magnet constructed by the Efremov Institute in St. Petersburg [Sta06] is shown in figure 4.25 as schematic. The shape of two superconducting Helmholtz coils cooled by a liquid helium bath is designed to ensure a homogeneous magnetic field parallel to the beam axis. The variation of the magnetic field inside the coils is less than 20%. The coils of the magnet are supported by a surrounding iron yoke which additionally attenuates the field outside of the RD to remaining 2mT in a perpendicular distance of 2m from the center. For an incident particle passing the SSD and the SFT several space points are available to reconstruct the momentum by the trajectory. If the particle stuck in the inner part of the RD the momentum can be reconstructed by CHAPTER 4. THE RECOIL DETECTOR 68 Figure 4.23: The concept of the Photon Detector. The setup of the layers partially projected on the downstreamed x-y-plane in the upper schematic. The lower part is the projection on the x-z-plane. As indicated by the se- quence of the gray tungsten preshower layer and the blue strip layer the upside part is in the inner part of the detector. (a) (b) Figure 4.24: Shape of the scintillating strips (a) The concept of the seg- mented strips and the wave-length shifters are shown. (b) Picture of the transparent stripe of the stereo layer and the green wave-length shifters in the background. CHAPTER 4. THE RECOIL DETECTOR 69 Iron Shielding Cryostat SC Coils SciFi Connector Plate Photon C3 Collimator Detector SciFi Si Detector Detector Cooling Silicon Si Detector Detector Connectors Target Cell Hybrid Flange 0.1 0.2 0.3 m Figure 4.25: Setup of the superconducting magnet together with the other parts of the Recoil Detector. the amount of deposited energy. 4.7 Installation of the Recoil Detector The Installation of the Recoil Detector into the HERMES experiment started in November 2005. At the beginning the equipment for the polarized target was removed to prepare the area in front of the HERMES forward spec- trometer. First the basement for the Recoil Detector and the liquid helium pipe for the superconducting magnet have been installed. Afterwards all the electronics and cabling were connected. The prepared Recoil Detector ready for data taking is shown in figure 4.26 right before the interlock of the restricted area was set. Due to the well prepared schedule and very focused activities the installation was very smooth and very successful. Afterwards the apparatus of HERA were switched on in order to start to study the machines with low current at the beginning as many of beam magnet coils have been replaced. The investigations and preparations of the storage ring succeeded very fast so the normal running conditions were established soon with ∼ 110mA protons at 920GeV and ∼ 43mA electrons at 27.6GeV . CHAPTER 4. THE RECOIL DETECTOR 70 Figure 4.26: The picture shows the installed Recoil Detector right before the interlock of the restricted area was set. HERMES based readout The readout system of the Recoil Detector by the HERMES DAQ consists of several steps. The first step is the readout of the digitizers to the internal shared memory by the DSP (Digital Signal Processor) of the VME crate controller. The control program for the DSP was written in assembler. The next step is to collect data from all the VME crates by the DSP of the master VME crate. A standard PC based readout transfers in the next step the data inside the shared memory of the master VME crate controller to the local memory and merges additional status informations to the data. In the final step all event based collected data are written to the tape. In case of the SFT additional debugging informations for each event about the internal state of the DTU and the RC are saved as well. Therefore additional cross check capability for the data quality in the offline analysis chain can be provided. Also slowcontrol data are important for later data analysis and detector recalibration. Chapter 5 Performance This chapter will focus on the SFT (Scintillating Fiber Tracker) which was the main contribution of the Giessen group to the RD (Recoil Detector). The other detectors are covered in detail in [Pic08] and in [Vil08] for the SSD (Silicon Strip Detector) and in [Haa07] for the PD (Photon Detector). First a short introduction about the bench tests like proof of concept, alignment run and the final cosmic ray test run will be given. Preliminary performance of the installed RD with the SFT in correlation with the HERMES forward spectrometer and first calculations of azimuthal asymmetries of hard exclu- sive reaction will be presented. 5.1 Proof of concept As shown before the SFT is one of the major components for particle iden- tification and track reconstruction. A series of simulations and measure- ments was done to proof the concept of the SFT. The required setup of the SFT proposed in [Kai+02] were designed by studying Monte Carlo simula- tions on pions and protons [Som03]. The measurements at GSI in Darm- stadt/Germany were done in 2003 to proof the chosen realization of the SFT and to settle down final configuration parameters [Hoe+05], [Hoe+07]. The used hardware for the readout are documented in [Rub06]. 5.1.1 Test beam environment u The SIS (Schwerionensynchrotron) at GSI (Gesellschaft f¨r Schwerionen- forschung) provides secondary beams over a wide momentum range gener- ated by a primary proton or ion beam delivered by the SIS colliding with a production target [Dia+02]. For each of the four scheduled test beam experiments the secondary beam consisting of pions and protons was gener- ated by a primary 12 C beam penetrating a 120mm thick B4 C target. The contamination by other particles was negligible or was excluded by parti- 71 CHAPTER 5. PERFORMANCE 72 f m S I S r o p r o d u c t i o n t a r g e t H c a v e C c a v e A c a v e c a v e d i s t a n c e A 6 3 m B B 8 9 m c a v e C 6 9 m H 3 3 m 0 3 0 m Figure 5.1: This is a schematic of the first floor of the target hall at GSI [Dia+02]. The primary beam of protons or ions entering at the top penetrate the production target to generate secondary beams which are led by beam lines partially covered by high magnetic fields for particle selection in a wide momentum range to several experimental locations. The test experiment for the RD especially for the SFT is located in cave A. CHAPTER 5. PERFORMANCE 73 cle identification. The selected central momentums of the secondary beam with a smearing of below 10% was 300M eV /c, 450M eV /c, 600M eV /c and 900M eV /c to cover the design requirement. The setup was located in Cave A in the target hall as shown in figure 5.1. 5.1.2 Detector prototype The SFT test module [Hoe+07] was built like a section of the SFT with two units representing an inner and an outer barrel each with a parallel and a stereo module as visualized in figure 5.2. The geometry was similar to the final SFT configuration. Each SFT module was connected via a light guide to a MAPMT. Figure 5.2: The test module used for the last test beam run in November 2003 [Hoe+07]. 5.1.3 Test beam setup The test beam setup [Hoe+07] shown in fig 5.3 consists of the three sub detector of the Recoil Detector, four plastic scintillators and a MWPC (Multi Wire Proportional Chamber). Two plastic scintillators S1 and S2 are used to provide a primary trigger for the digitizing and readout. By combining this primary trigger with a third plastic scintillator S0 time of flight information was available to provide particle identification. The fourth plastic scintillator S3 was needed to study the efficiencies of the detectors. To monitor the beam position a MWPC was available as a reference system to study the resolution of the prototypes. The trigger logic was based on standard VME module while the readout was done with a standard PC. The electronics of the SFT readout [Hoe+05] as shown in figure 5.4(a) was tested as well. 5.1.4 Test beam results Several test beam runs were carried out to investigate and to proof the concept of the detector. The major results will be presented. CHAPTER 5. PERFORMANCE 74 Figure 5.3: The test beam setup [Hoe+07] used for the last test beam run in November 2003. The SSD (red), SFT (blue) and PD (green) are combined. S1 and S2 are used for trigger generation while S3 is used for efficiency studies. S0 is used for PID by time of flight separation. MWPC are used as a spatial reference of the combined π − p beam in the momentum range between 300M eV /c to 900M eV /c which is entering from the left. ADC spectra Typical raw ADC spectra [Hoe+07] is shown in figure 5.4(b). The pedestals stays at the raw ADC value around 925, while the larger signal are repre- sented by smaller raw ADC values due to hardware implementation. Particle identification For particle identification [Hoe+05] the deposited energy and the time of flight informations are combined. As shown in figure 5.5 the particle sepa- ration between pions and proton for momentums around 300M eV /c is very clean while for 900M eV /c the areas getting close and a small contamination of heavier particles can be recognized. Energy response The energy response [Hoe+05] of the leading fiber is shown in figure 5.6(a) for pions and protons with several mean momentums. The corresponding Monte Carlo simulations done with GEANT [GEA93] for comparison are visualized in figure 5.6(b) and in good agreement with the measurements. Efficiency If NRef is the number of events with a hit in the reference plastic scintillator and if NSF T is the number of events seen by the reference and the SFT, the CHAPTER 5. PERFORMANCE 75 (a) (b) Figure 5.4: (a) Picture of the electronics of the SFT readout. (b) Raw ADC spectra for PMT response to a LED induced light. The contributions of single, two and three photons are shown [Hoe+07]. N.B. due to hardware implementation higher signals has lower raw adc values. Here the pedestal stays at a raw adc value of 925. 8000 7000 dE (ADC channel) dE (ADC channel) 7000 6000 6000 5000 5000 4000 4000 3000 3000 2000 2000 1000 1000 0 0 1860 1880 1900 1920 1940 1960 1980 80 100 120 140 160 180 200 220 TOF (TDC channel) TOF (TDC channel) (a) (b) Figure 5.5: The particle identification is for 300M eV /c (a) well separated while the response for 900M eV /c (b) are closer and some few heavier parti- cles can be detected [Hoe+05]. efficiency is defined as the ratio ǫ = NSF T /NRef . In figure 5.7 the efficiency ǫ for protons and pions is given for the investigated momentums and they CHAPTER 5. PERFORMANCE 76 counts [arb. units] counts [arb. units] 1600 pions 0.07 pions protons, 900 MeV/c protons, 900 MeV/c protons, 600 MeV/c 1400 protons, 600 MeV/c 0.06 protons, 450 MeV/c protons, 450 MeV/c protons, 300 MeV/c 1200 protons, 300 MeV/c 0.05 1000 0.04 800 0.03 600 0.02 400 0.01 200 0 0 0 10 20 30 40 50 60 70 80 90 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 deposited energy [p.e.] deposited energy [MeV] (a) (b) Figure 5.6: The leading fiber response for real data (a) and with GEANT Monte Carlo simulation (b) for different mean momentum of protons [Hoe+05]. are determined to be above 98% for a module. (a) (b) Figure 5.7: The efficiency for pions (a) and protons (b) versus the mo- mentum is shown for different threshold level in units of photon electrons [Hoe+07]. The efficiency is independent of this signal cut off above 98%. The values for protons are shifted for better visibility to higher momentum. CHAPTER 5. PERFORMANCE 77 Readout implementation Inside HERA at DESY four experiments have to cooperate. Therefore an access to the restricted area inside the interlock area is not trivial and the possibility for a repair or a hardware exchange is very limited. This forces to have an very robust readout as the readout electronic will stay inside the restricted area. Thus tests of the SFT readout had to be done. As shown in figure 5.8 some small problems occurred during the test beam runs [Hoe+05]. The error detection and error recovery have to be improved by storing the status informations into the data stream. (a) (b) Figure 5.8: Multiplicities of the clusters sizes in units of pixels [Hoe+05]. (a) A bunch of entries in the spectrum of the multiplicity of cluster sizes are due to problems in the readout of the SFT electronics. (b) These strange events are clearly located in some certain period of data taking and are ex- cluded from further evaluations of the SFT. The other issue is the handling of the readout. During these tests the readout was running in the so called 'stop mode' [Rub06]. All the handshake signals for the readout had to be generated by the readout software running on a standard PC which limits the readout to an accepted trigger rate of around 100Hz. In contrast to that in the HERMES experiment a trigger rate of several hundreds Hz during the high density runs or at the beginning of the fill is not unusual. Therefore the readout of the SFT had to be extended to the so called 'run mode'. In this readout mode most of the handshake signal are generated by the hardware itself and several informations about the internal state are available for error detection. It's implementation was one of the main topic of this work. CHAPTER 5. PERFORMANCE 78 Conclusion for the testbeam run The test beam experiment was very successful to improve the understand- ing of the components, to settle down final configuration parameter and to provide reference data and plots to tune further setups. 5.2 Alignment run The intermediate test run for the SFT was the alignment run [Ste+05]. One aim was to implement the readout named 'run mode' and to test it under real conditions. The measurement of the SFT internal alignment for the later space point reconstruction of each fiber was the second goal of this alignment run. 5.2.1 Environment at test beam 22 The test beam areas at DESY II [Beh+07], [Gre+04] shown in figure 5.9 provide a e− or e+ -beam up to 7GeV beam energy. Accelerated electrons or positrons stored in DESY II generate a primary γ-ray by Bremsstrahlung on a 7µm carbon fiber. A metal plate of selectable material and thickness is used to convert the γ-ray by pair production into electrons and positrons. The leptons are bend due to a dipole magnet behind the converter. A set of collimators are used to form the extracted lepton beam before it enters the test beam areas. As the setup is very simple the desired beam energy and particle types can be selected by varying the converter properties and the magnetic field behind the converter. 5.2.2 Setup of the alignment run The figure 5.10 illustrates the setup of the alignment run. The Zeus tele- scope consisting of three silicon strip detector. It was used as reference measurements of the properties of the lepton beam. The plastic scintillators S1 and S2 are used to generate a trigger signal for digitizing and readout. The active size of the reference telescope is ∼ 32x32mm2 . As shown, the axis of the barrel of the SFT is turned to vertical direction and is mounted on a rotational frame sitting on a movable table. This gives the opportunity to rotate and shift the barrel accordingly to measure the whole detector step by step. 5.2.3 SFT readout in 'run mode' For the data taking a readout of the Zeus reference telescope running with LabVIEW was available. A first readout of the SFT was implemented in this framework too. In order to minimize the necessary amount of time to scan CHAPTER 5. PERFORMANCE 79 Collim Magnet ator Converter Fiber γ e+ e+ − /e e− e+ e− Spil Counter DESY II Figure 5.9: Accelerated electrons or positrons stored in DESY II generate primary γ-rays by Bremsstrahlung. These γ-rays are converted by pair pro- duction to electrons and positrons of a wide range of momentum. The dipole magnet behind the converter provide momentum separation of the secondary beam by deflection to a flat fan out. A set of collimators forms the final beam of selected momentum range and particle type. [Beh+07] the whole detector a standalone console program without the framework was developed with a gain in speed of more than one order of magnitude. During the alignment run the SFT readout using the run mode of the readout controller was implemented and tested. Small hardware modifica- tions had been made to terminate the signals properly and to provide the few essential handshake signals. Due to these simplifications the readout has been speed up to read unsparsified data with an accepted trigger rate of several hundred Hz. Technical details can be found in [SFT+06]. 5.2.4 Alignment run results Zeus telescope internal resolution The hits in the three silicon detectors are used to reconstruct the projectile trajectory by combining these space points. The internal resolution of the reference system with the SFT in front is shown in figure 5.11. The resolution for the middle silicon sensor is around 20µm while the resolution without the disturbance of the SFT is around 19µm. This gives a possibility to measure the path of the fibers very precisely. CHAPTER 5. PERFORMANCE 80 Figure 5.10: Setup of the alignment run [Ste+05] with the SFT detector in front of the Zeus telescope with the three silicon sensors and two plastic scintillators S1 and S2 for trigger generation. The beam enters the area from the right. The upper part is the side view while the lower part shows a top view with the achieved resolutions. Reconstruction of a single fiber The reconstruction of the fibers are done in two steps. First single measure- ments are used to determine the short paths of the fibers in the clipping area. The reconstruction of a single fiber is shown in figure 5.12. The gath- ered positions are fitted with an gaussian curve visualized as a blue curve wherein the intrinsic fiber distribution and the distributions of the telescope are folded together. The contribution by the Zeus reference telescope itself is shown as a red curve. As the fiber diameter is 1mm the resolution of the reconstructed fibers around 400µm is very good. Database of fiber positions In the second step the results of each clip are combined by adjusting the overlap region to built a global 3D-picture of the SFT detector. For further CHAPTER 5. PERFORMANCE 81 Figure 5.11: The internal resolution of the Zeus telescope [Ste+05] is shown for the x direction (upper) and for the y-direction (lower). The sigma of each plot is 20µm. Without the disturbance of the SFT detector the sigma is 19µm. usage the paths of the fibers are described by polynominals up to fourth degree as shown in figure 5.13. These informations are afterwards used to align the SFT barrels to the other subdetectors of the Recoil Detector. Once the internal alignment is done elastic scattering events can be used to align the Recoil Detector to the HERMES forward spectrometer. The final fiber position resolution of 280µm is very close to the theoretical resolution of 220µm for a fiber with 1mm diameter [Yas07]. Final SFT readout In order to include the SFT readout into the HERMES DAQ an DSP (Dig- ital Signal Processor) have to be used. The used DSP is a ADSP 21061L SHARC-processor from Analog Devices [AD08b] which are based on a Har- vard architecture where data and code are in separate memory space. The DSP has to be programmed in native assembler due to execution speed as a compiler based program has a high framework overhead. The alignment run CHAPTER 5. PERFORMANCE 82 Figure 5.12: Reconstruction of a single fiber position together with the fit as blue line and the contribution of the intrinsic resolution of the Zeus telescope as red is shown. [Ste+05] showed that the unsparsified readout speed of the SFT will be most likely not sufficient in all cases for the final implementation which will use twice the number of channels to be read out. In order to be on the safe side in any case concerning available data rate and to simplify the readout of the SFT it was decided to implement the final SFT readout with the support of the DTU (Detector Trigger Unit). Conclusion for the alignment run This second bench test was very successful. The intermediate readout was implemented, tested and running very stable. In total more than 600 runs with each 100k events had been taken. The paths of the fibers was measured very precisely. CHAPTER 5. PERFORMANCE 83 Figure 5.13: The trajectory of the reconstructed fibers are described by polynominals up to fourth degree which are visualized by different colors. The boundaries of the SFT modules are around y=22500 as visible on the right side. [Ste+05] 5.3 Cosmic ray test run In 2005 the cosmic ray test run as shown in figure 5.14 was performed to prepare the installation of the RD into the Hermes Detector environment. For this an area in the east hall close to the HERMES detector but out- side of the interlock area was established to perform extended stress tests. The RD was put as close as possible to the final setup. The supercon- ducting magnet was installed and tested under the experimental conditions. The readout of the RD was implemented and prepared to be merged into the HERMES DAQ together with the slowcontrol chain, which collects sec- ondary informations of the detectors and the condition of HERA running. The setup was ready for data taking in March 2005. 5.3.1 Readout of the SFT During the previous test runs the readout for the SFT was improved step by step [SFT+06]. The change from stop mode to run mode increased the accepted trigger rate by one order of magnitude. A further improvement was possible with the use of the DTU which controls the signal flow over the backplane. This VME based device generates the handshake signals by a SM (State Machine) realized with a FPGA. A SM is a model of a machine CHAPTER 5. PERFORMANCE 84 Figure 5.14: The complete installation of the cosmic ray test run setup consists of the Recoil Detector with the superconducting magnet in the cen- ter, the pump stand (front right), the magnet control system (front left), the platform with the rack for the readout electronic and the PMT wall of the SFT (left to RD) and the barely visible readout racks of the other RD components in the background. where state transitions and actions are performed by a set of rules stored in the internal memory. Additional debugging facilities were implemented into the SM as well. They provide informations about internal counters and tagging of events in order to detect desynchronisation during readout and to be able to resynchronize them if desired. During the cosmic ray test run some malfunctions in the readout have been detected. It turned out that the timing of the signal flow in the PFM (Preprocessing Frontend Module) was slightly disturbed. With extensive stress tests the problem was found to be caused by the design of the back- plane shown in figure 5.15(upper) which were used. The layout of the con- ducting paths was done with an auto router which took no informations about the purpose of a path into account. The paths can have significant different path length and therefore different propagation times. The power lines have the same small width as the signal lines but drives much higher currents. Therefore the voltage drop was too high to keep the four PFM in a stable operational state under all running conditions. Under heavy load the voltage drop is that high that the signal propagations are partially delayed CHAPTER 5. PERFORMANCE 85 due to longer rise time and thus edge detections so that the timing is not matching properly anymore. After these observations the backplane was completely redesigned keeping only the locations of the connectors as shown in figure 5.15(lower). The routing was done manually. It turned out that the new backplanes and the previous modification of the readout controlled by the DTU the unsparsified readout of the SFT was now running with more than 1kHz accepted trigger rate very fast and very stable. Figure 5.15: A backplane with two layers of the first production (upper) where the designing and the routing of the conducting paths were made by the auto router. Significant different conducting path lengths and voltage drop over the undersized power lines causing improper timing thus readout malfunctions under high load. A sample of the new production with four layers and with hand made routing is shown in the lower half. 5.3.2 Recoil Detector readout From April 2005 until the shutdown of the cosmic ray test run in August 2005 a large amount of data where taken. The local Data acquisition was CHAPTER 5. PERFORMANCE 86 prepared for two major modes, either connected to the HERMES DAQ or in a standalone mode. The standalone mode is split into four sub modes. • pedestal mode In this mode all available channels of all detectors were read out by self generated trigger. These data are used to measure the signal background. • gain monitoring mode In order to monitor the gain of the MAPMT this readout mode where established and running unsparsified too. Similar to the pedestal mode a trigger was generated to start the digitization. in addition this signal also triggers a light source to imprint a certain amount of light led by light guides into the detector and into the reference PMT for cross check. • reference MAPMT readout mode An alpha source inside the reference PMT housing was used to gener- ate a signal which is used to trigger the digitization and the unspar- sified readout. These signals are used to monitor the stability of the gain monitoring system of the MAPMT. • cosmic ray data taking mode Cosmic muons where used to generate a trigger when a particle is passing through the detector. By combining certain strips a dedicated cosmic ray trigger could be provided. This mode was used to study calibrations, alignments, track reconstructions and efficiencies of the RD. While all the preparations and tests were carried out more than 1G events (2TB data) were taken. The results from cosmic ray test run will be dis- cussed in next sections. 5.3.3 Superconducting magnet The setup for the superconducting RD magnet was prepared as well. For this an area was foreseen to hold the vessel with liquid helium. Even with the limited vessel volume the magnet was running for several hours to test the compatibility of the used hardware to the surrounding magnetic field. Another important issue is the preparation and test of the track finding, the reconstruction algorithm and the implemented routines as charged particles are deflected in the magnetic field. A detailed documentation about the superconducting magnet can be found in [Sta06]. CHAPTER 5. PERFORMANCE 87 5.3.4 Cosmic ray test run results During the cosmic test run many stress tests were carried out to check possible design flaws or missing demands. As the attention for data taking and data preparation switched to the real production only few results are gathered yet. Hit distribution for cosmic ray events The major cosmic ray trigger setup was done by combining the lower half of the strips of the PD. As most muons are passing from upside to downside this setup ensures the particle passes through the detector as it covers the width of the detector. In figure 5.16 the hit distribution for the inner parallel SFT layer is shown. As the trigger is generated by the lower half of the PD the hit probability for the lower half are slightly higher than for the upper part. Therefore the slight asymmetry in the multiplicity is expected. As a straight vertical passing particle has a higher probability to hit several fibers per event at the side than in the middle of the barrel the two large dips are expected as well. Realignment The inner parallel layer consists of 21 SFT-modules with 64 fibers while the outer parallel layer consists of 18 modules with 128 fibers. As the light guides and within this the according MAPMT are counted clock wise starting at the top the fibers in the inner and outer parallel layers have an intrinsic offset. To see the correlation between the parallel layers the hits are plotted in figure 5.17 versus fiber numbers in each layer. The bisectioning line has the expected small tilt as a straight cosmic ray line enters the inner layer under a different angle than the outer layer. The mean value of the distribu- tion projected perpendicular to the bisectioning line measures the relative rotation. The virtual relative rotation for the minimum mean value gives the relative of the layer to each other and confirms very well the expected value. Conclusion for the cosmic ray test run The cosmic ray test run was an important and very successful intermediate step to install the Recoil Detector. Few design flaws like broken hardware, some timing issues in the electronics and the cabling scheme of the light guides for example has been gathered and solved. After the partial dis- assembly some broken components were replaced so that the RD and the corresponding components were prepared to be installed during the winter shutdown in 2005. CHAPTER 5. PERFORMANCE 88 Figure 5.16: Hit distribution for two space point tracks for cosmic rays in the inner parallel SFT layer. The fiber 1 and 1096 are at top position downstream with clockwise counting. The trigger generation by the lower half of the PD is the reason for the asymmetry in the hit count for the upper (top around 1) and lower half (bottom around 550). The hit count around fiber 300 and around fiber 900 is due to higher hit multiplicity of vertical passing particles per event . 5.4 HERMES experiment run The Installation of the RD during the winter shutdown of HERA in 2005 into the HERMES detector was smooth and successful. Some results of the experimental run will be discussed in this section. CHAPTER 5. PERFORMANCE 89 Figure 5.17: Cross check of the SFT internal alignment. On the left side the raw fiber hits in the inner versus the outer parallel layer are plotted. On the right side the mean value of the projection of these hits by shifting certain number of fibers is shown and confirms the known offset. 5.4.1 Production A short introduction about the available productions and how they are gen- erated will be introduced. Data taking The HERMES Detector was upgraded during the winter shutdown in 2005 with the Recoil Detector, the slowcontrol branch was adjusted and the read- out of the Recoil Detector was merged into the HERMES DAQ. At the beginning an electron beam was used until the mini shutdown (June 2006) followed by a positron beam until the end of HERA. At the end of HERA running the protons energy were changed twice to make additional stud- ies in different kinematic regions for collider experiments H1 and ZEUS. A summary of available runs is given in table 5.1. The whole data taking interval of the RD can be separated into five periods. 1. From start to mid of March The RD was fully assembled. The SFT was running from the begin- ning. As found later neither the PD nor the SSD was fully operational until the reassembly of the RD. Furthermore the SFT-TDC might be desynchronized due to a error in the DSP assembler code which can be resynchronized again as tagging informations are available. CHAPTER 5. PERFORMANCE 90 projectile e− electrons e+ positrons Eproton /GeV helicity in 2006 −1 ←→ 1-10958 920 +1 →→ 10959-14244 14245-43990 920 −1 ←→ 43991-48195 920 helicity in 2007 −1 →→ 1-17981 920 +1 ←→ 17982-34735 460 +1 ←→ 34736-40840 575 Table 5.1: List of runs of accumulated data samples with Recoil Detector helicity states parallel (+1 →→) and anti-parallel (−1 ←→) to the beam direction. At the end of HERA running the energy of the protons were varied for studies in different kinematic regions for collider experiments H1 an ZEUS. HERMES was running with electron and positron beam only but the background might be dependent on proton beam energy. 2. From mid of March to beginning of May The background signal in the SSD jumped suddenly clearly to higher contribution. During the maintenance access beginning of May some collimators were removed to look inside the target area. A burned hole was found as most likely reason for the sudden increased electrical noise in the SSD. Unfortunately during the reassembly of the target cell was crunched. This was the cause for very high parasitive radiation while trying to get HERA running again and therefore part of the SSD was damaged by these radiations. In order to repair the SSD it was removed. 3. From beginning of May until end of June The SSD was absent for repair while PD and SFT was partially reca- bled up again for some studies and fixing all the remaining problems. 4. from end of June to beginning of September The complete RD was reassembled and under commissioning again. 5. from September to the end of HERA The RD enters the fully operational phase and running smooth even after the shutdown of HERA for cosmic ray data taking for additional alignment and calibration. During the data taking with electron beam only the SFT part of the RD was operational. As the additional space points of the SSD are not available the resolution of the reconstructed track with only the SFT will drop in the low momentum region. On the other side the extrapolation of CHAPTER 5. PERFORMANCE 91 kinematic variables in the limit t → 0 will be difficult as the SSD covers the low momentum region. Therefore the analysis of the electron data of the Recoil Detector for physical content will be challenging. Preparation of data The raw data taken with the HERMES DAQ [Her+03] was stored on the tape robot in the EPIO (Experimental Physics Input Output) format [McL+93]. This EPIO format was designed at CERN. Processed data productions are stored in the DAD (Distributed ADAMO Database) [Wan+95] format which was developed by the HERMES collaboration. The DAD format using the entity-relationship database of ADAMO (ALEPH data model) [ADA95] as underlying layer. In order to provide data for physical analysis a two level data pro- cessing were created at HERMES. The first process called HRC(HERMES reconstruction)-production is used for studies of the detectors and for deter- mine configuration data like calibration, alignment and other time depen- dent properties. The data of the first process are merged afterwards together with the slowcontrol data by the second process called DST(Data Summary Tape)-production in order to provide high quality data for physics analysis. In order to generate the HRC-productions the raw data from the tape robot are piped through several level of post processing routines. The first major task is to translate informations like rack numbers, crate numbers, module numbers, channel numbers to geometrical information like coordi- nates upon a time based lookup table. In the second stage the raw signal values are translated to informations like energy depositions or timestamps. The third stage is combining these information to reconstruct tracks. In a first step short tracks within sub detectors are searched and in a second step these short tracks are combined to long tracks which covers the complete trajectory of a particle. Due to characteristic behavior of particles the re- sponse in the sub detectors are combined to give likelihoods of the detected particle. Finally the extracted informations are used to calculate the pa- rameters for alignment and for calibration and to determine the efficiencies of the production. Investigations made with these productions are used to gather parameters for the next iteration. The HRC-production follows the naming convention YYv where YY are the two least significant decimal digits of the year and v the version of the production. For the DST-production the data stream of the slowcon- trol branch are merged with the HRC-production. Due to this the naming convention for the DST-productions YYvr are correlated to the under- lying HRC-production with r as the additional revision number. These DST-productions are now used to physics analysis. Nevertheless important informations for the HRC-productions can be gathered by comparing DST- productions of different years. CHAPTER 5. PERFORMANCE 92 As all the time new algorithms and research tools are implemented and codes are bug fixed. As the understanding in detector calibration reaches saturation some versions of production are frozen and physics anal- ysis starts. For the following overviews the HRC-production 06d and the DST-production 06d0 were used which was released recently. For investiga- tions with the Recoil Detector the track reconstruction method 7 was used as the final tracking method is not settled down yet. Especially the usual event selections are applied to be comparable to other analysis in this field. More investigations and results on previous productions with the Recoil Detector are documented in [Vil08] and constantly updated on [Her+08]. 5.4.2 Overview of elastic scattering events Elastic scattering of electrons or positrons on protons is a clean and simple way to study the intersection of the whole HERMES forward spectrome- ter with the Recoil Detector. In order to select an elastic scattering event few criteria are requested. The event has to have exactly one track of the scattered beam particle. The energy of this particle must be above 25.5GeV and below the beam particle energy. The starting vertex have to be between 0cm and 25cm in z-direction which is along the beam axis and at a distance below 0.75cm to the beam axis. Some geometrical properties measured by the HERMES forward spec- trometer are compared with those measured by the Recoil Detector. The vertex reconstruction in z-direction ( see figure 5.18) of the scattered beam lepton by the HERMES forward spectrometer shows a clear correlation to the vertex of the recoiled particle reconstructed with the data available by the Recoil Detector and the difference of these values are narrow. The reconstructed polar angle (see figure 5.19) is a weaker crosscheck as the distribution by the HERMES forward spectrometer is smeared due to the large level arm to the calorimeter. Nevertheless the transverse components of the polar angle are in good agreement. The azimuthal angle (see figure 5.20) reconstructed by the spectrometer has to have the opposite direction to the one reconstructed with the RD due to momentum conservation. As the geometrical component parallel to the beam axis is not involved the adjusted azimuthal angles has very narrow correlation. The width of the distribution is almost as small as the expected azimuthal resolution of the SFT. In summary, even in this early stage the results are already in promising good agreement with previous productions and with expected performance. 5.4.3 Overview of Deep Inelastic Scattering events Another set of events for cross check of the performance of the detectors are deep inelastic scattering processes. In this case only the scattered beam CHAPTER 5. PERFORMANCE 93 htemp counts [1] z vertex position RD [cm] Entries 159367 350 30 10000 Mean 0.3081 RMS 2.783 300 25 8000 20 250 15 200 6000 10 150 4000 5 100 0 2000 50 -5 0 0 0 5 10 15 20 25 -30 -20 -10 0 10 20 30 z vertex position FS [cm] z vertex position RD - z vertex position FS [cm] (a) (b) Figure 5.18: The correlation for elastic events between HERMES forward spectrometer (FS) and Recoil Detector (RD) for the vertex in z-direction is in good agreement. (a) 2D-plot of vertex position along the beam axis. (b) Distribution of the differences. transverse polar angle RD [rad] 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.04 0.045 0.05 0.055 0.06 0.065 0.07 0.075 0.08 transverse polar angle FS [rad] Figure 5.19: Correlation for elastic events between HERMES forward spec- trometer (FS) and Recoil Detector (RD) as 2D-plot of transverse polar angle θ component is smeared due to limited resolution in the HERMES forward spectrometer. Few remaining background is visible. particle has to be detected in the final state and given therefore an simple probe. For the event selection a designated bit pattern (0x441e1bce) [Her+08] of the data quality was used which contains general informations about the CHAPTER 5. PERFORMANCE 94 azimuthal angle RD [rad] htemp counts [1] 3000 Entries 159367 Mean 0.1033 30000 RMS 0.8221 6 2500 5 25000 2000 4 20000 1500 3 15000 1000 2 10000 1 500 5000 0 0 0 0 1 2 3 4 5 6 -6 -4 -2 0 2 4 6 azimuthal angle FS [rad] azimuthal angle RD - azimuthal angle FS [rad] (a) (b) Figure 5.20: Correlation for elastic events between HERMES forward spec- trometer (FS) and Recoil Detector (RD) for the azimuthal angle is in a very good agreement. (a) 2D-plot of azimuthal angle. (b) Distribution of the differences. The small peak is as small as the fiber diameter of the SFT. rating of the data. Furthermore the usual and extended fiducial volume cuts were applied to ensure that detected and scattered beam lepton passes from the interaction region in the target cell to the electromagnetic calorimeter. The vertex has to full fill 5 < V ertexZ/cm < 20 for the z-direction and has to have a smaller distance from the beam axis than 0.75cm. For the following kinematic variables the requirements to ensure DIS regimes are listed in table 5.2 variable unit lower limit upper limit x 1 0.03 0.35 Q2 (GeV /c)2 1 W2 (GeV /c)2 9 ν GeV 22 Table 5.2: List of some kinematic cuts for deep inelastic scattering event. The distributions of the usual observables for the 06d0 productions are summarized in figure 5.21. The triangle shape of the reconstructed vertex in z-direction is in good expectation. The gas target is injected at z = 12.5cm with the highest density and are pump out at the two ends of the target cell. The shape of the distribution of the vertex distance to the beam axis are in- dicating a close to beam axis interaction area as well. The φ-distribution of the scattered beam lepton reflect the upper and lower half of the HERMES CHAPTER 5. PERFORMANCE 95 forward spectrometer very well. Together with the θ-distribution of the scat- tered beam lepton the acceptance of the forward spectrometer is indicated as well. The distributions of the transfered momentum fraction and the transfered energy fraction is as expected too. In total, these observables are in a good agreement with previous data productions. 5.4.4 Overview of DVCS events For the event selection of DVCS additional requirements are needed in order to ensure exclusivity. The event has to consists of exactly one scattered beam lepton and exactly one real photon. The momentum transfer Q2 due to the scattered beam lepton has to be below 10GeV 2 . Beside the fiducial volume cut for the real photon the energy deposition in the H2 preshower must be above 0.001GeV while the energy deposition inside the electromagnetic calorimeter must be above 3GeV . The calculated angle θγ ∗ γ between the virtual photon and the produced real photon has to be in the range between 5mrad and 45mrad. The constrained four momentum transfer tc −Q2 − 2ν ν − ν 2 + Q2 cos (θγ ∗ γ ) (5.1) tc = 1 1+ Mp ν− ν 2 + Q2 cos (θγ ∗ γ ) between initial and final nucleon of the momentum transfer to the target calculated in the laboratory frame with the missing invariant mass value must be above −0.7GeV 2 . The squared missing invariant mass Mx has to2 be in the range −2.25GeV 2 to +2.89GeV 2 . The distributions of the usual kinematic observables are shown in figure 5.22. The distribution of the squared four momentum transfer Q2 by the scattering beam lepton reflect the constraints to have more statistics but to ensure factorization. The center of mass distribution of the system consisting of the real photon and the recoiling target has the required limitation of W 2 > 9GeV 2 . The restriction for the transfered momentum fraction x to the target limits the correlated transfered energy fraction y accordingly. To ensure the selection of deeply virtual Compton scattering from Bethe- Heitler process the angle between the virtual photon to the real photon are limited accordingly while the distribution of the energy of the real photon are intrinsic inside the boundaries. In figure 5.23 the distribution of the squared four momentum transfered to the target with respect to the missing invariant mass is shown. To reject background exclusivity the events of the exclusive regions are requested. The pure calculation of the squared four momentum transfer with the proton mass for a hydrogen target achieves to a broad distribution around zero. This does not reflect the true kinematics. By using the missing invariant CHAPTER 5. PERFORMANCE 96 ×10 ×10 counts [a.u. 1] 3 3 counts [a.u. 1] 250 700 600 200 500 150 400 100 300 200 50 100 0 0 4 6 8 10 12 14 16 18 20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 z vertex position [cm] vertex distance [cm] ×10 ×10 3 3 counts [a.u. 1] counts [a.u. 1] 700 250 600 200 500 400 150 300 100 200 50 100 0 0 0 1 2 3 4 5 6 0.05 0.1 0.15 0.2 0.25 vertex azimuthal angle [rad] vertex polar angle [rad] ×10 ×10 3 3 counts [a.u. 1] counts [a.u. 1] 450 220 400 200 180 350 160 300 140 250 120 200 100 80 150 60 100 40 50 20 0 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x [1] y [1] Figure 5.21: General kinematic variables of the HERMES forward spec- trometer gathered by measuring the scattered beam lepton only in good agree- ment with previous data productions. CHAPTER 5. PERFORMANCE 97 counts [a.u. 1] counts [a.u. 1] 1200 500 1000 400 800 300 600 200 400 100 200 0 0 2 4 6 8 10 10 15 20 25 30 35 40 Q^2 [GeV^2] W^2 [GeV^2] counts [a.u. 1] counts [a.u. 1] 900 450 800 400 700 350 600 300 500 250 400 200 300 150 200 100 100 50 0 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x [1] y [1] counts [a.u. 1] counts [a.u. 1] 400 500 350 300 400 250 300 200 150 200 100 100 50 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 5 10 15 20 25 angle virtual photon to real photon [rad] real photon energy [GeV] Figure 5.22: General kinematic variables of the HERMES forward spec- trometer gathered for DVCS events in good agreement with previous data productions. CHAPTER 5. PERFORMANCE 98 mass to calculate this observable the shape looks like expected and is below zero as a space like property. Figure 5.24 shows the different shape in the distributions of the vertex distance for the 06b2 and for the 06d0 data production. After the realign- ment for the 06d production the distribution is closer to the beam axis. Further studies of the impact of the realignment is currently under investi- gation. As result, the general observables agree very well to those of previous productions as well. 5.4.5 Overview of Azimuthal Asymmetry Events which fulfill the requirements of hard exclusive processes are used to calculate the azimuthal asymmetries. During the data taking with the RD the gas target was unpolarized, thus beam spin asymmetry and beam charge asymmetry can be accessed. As mentioned before the latest data production 06d/06d0 was used to access these azimuthal asymmetries. The distributions for the missing invariant mass shown in figure 5.25 indicate small beam charge dependency for the exclusive part but not for the broad semi-inclusive region. The reason might be a not yet perfect calibration of the calorimeter which is currently under investigation. The wavelike deviation in the angle φ between the scattered plane and production plane as shown in figure 5.25 is not taken into account as well. As the calculations of the azimuthal asymmetries are not yet cross checked the data points are not fitted. Beam Spin Asymmetry In case of beam spin asymmetry the selected events are sorted by the helicity of the beam projectiles. The values plotted in figure 5.26(a) are determined by calculation of 1 N+ − N− (5.2) ALU = < PB > N + + N − whereas ALU is the Amplitude for longitudinal polarized beam (L) and unpolarized target (U ) while N + and N − are the relative numbers of events with parallel and anti-parallel beam helicity. The beam polarization is taken into account by the averaged beam polarization < PB >. The error bars are larger than expected by previous analysis [Vil08] while the shape roughly follows previous results. CHAPTER 5. PERFORMANCE 99 ×10 3 htemp counts [a.u. 1] Entries 2.765242e+07 450 400 350 300 250 200 150 100 50 0 -5 0 5 10 15 20 25 30 35 missing invariant mass [GeV^2] (a) counts [a.u. 1] counts [a.u. 1] 1600 1400 1400 1200 1200 1000 1000 800 800 600 600 400 400 200 200 0 0 -1.5 -1 -0.5 0 0.5 1 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 -0 t [GeV^2] tc [GeV^2] (b) (c) Figure 5.23: Distribution of the invariant missing mass are shown in (a) after applying all cuts for DVCS event selection. For exclusive region the range from −2.25GeV 2 to +2.89GeV 2 is assumed. Distributions of squared four momentum transfer to the hydrogen target with constant proton mass calculation (b) and with missing invariant mass calculation (c) for the ex- clusive region which is in good agreement with previous results. CHAPTER 5. PERFORMANCE 100 ×10 3 counts [a.u. 1] 700 3500 600 3000 500 2500 400 2000 300 1500 1000 200 500 100 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VertexDistance vertex distance [cm] (a) (b) Figure 5.24: Distributions of the vertex distance for the 06b2(a) and for the 06d0(b) [Vil08] data productions for comparison. The shift to lower values are due to change in the HERMES internal coordinate system. counts [a.u. 1] counts [a.u. 1] 0.018 300 0.016 250 0.014 0.012 200 0.01 150 0.008 0.006 100 0.004 50 0.002 0 0 -5 0 5 10 15 20 25 30 35 -3 -2 -1 0 1 2 3 missing invariant mass for e+ [GeV^2] angle scatter plane to production plane [rad] (a) (b) Figure 5.25: Small side effects currently under investigation. (a) Small shift in the response by the calorimeter for electrons (blue) and positrons (red) for missing mass distribution for the exclusive region. (b) Wavelike differences in angle distribution between scattering plane and production plane. Beam Charge Asymmetry In case of beam charge asymmetry which is unique at HERA, the exclusive events are sorted by the charge of the beam particles and the values plotted CHAPTER 5. PERFORMANCE 101 ALU(φ) counts(φ) [a.u.] 0.2 3700 0.1 3600 0 3500 -0.1 3400 -0.2 3300 3200 -0.3 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 φ [rad] φ [rad] (a) (b) Figure 5.26: (a) φ dependency of the leading amplitude of the beam spin asymmetry. (b) Differences in available data per bin in φ can lead to small deviations. in figure 5.27 are calculated with respect to N+ − N− (5.3) AC = N+ + N− whereas AC denotes the amplitude of the beam charge asymmetry. The relative number of events with positrons or electrons are N + and N − . The error bars and the shape of the curve are closer to previous results [Vil08]. 5.4.6 Overview of SFT response One of the key contribution of the SFT is the particle identification. By combining the reconstructed momentum of the recoiled particle to the energy deposition particle separation is possible. The correlation between these parameters is shown in figure 5.28. Events with required two space points in the SFT - one in the outer barrel and one in the inner barrel - show a separation between pions and protons with a small overlap region. By requesting events with four space points - two in the SSD and two in the SFT - a clear separation between pions and protons is possible and a likelihood based particle identification might be implemented in the future as well. 5.4.7 Overview of improvements by the Recoil Detector The contribution to improve exclusivity of DVCS samples by the Recoil Detector is shown in this section. As reminder the reaction of DVCS events can be expressed by CHAPTER 5. PERFORMANCE 102 AC(|φ|) counts(|φ|) [a.u.] 3700 0.05 -0 3600 -0.05 3500 -0.1 3400 -0.15 3300 -0.2 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 |φ| [rad] |φ| [rad] (a) (b) Figure 5.27: (a) |φ| dependency of the leading amplitude of the beam charge asymmetry. (b) Differences in available data per bin in φ can lead to small deviations. (5.4) l + p → l ′ + γ + p′ where l and l′ are the initial and scattered lepton, γ is the generated real photon and p and p′ are the initial and scattered proton. The three-momentum p′F S of the previously undetected scattered proton is calculated with data measured with the HERMES FS (forward spectrome- ter) by the missing invariant mass technique. With the RD (Recoil Detector) the three-momentum p′RD of the recoiling proton is measured directly. The correlation of these three-momenta is shown in figure 5.29. On the left in figure 5.29(a) the difference of the x-component versus the difference of the y-component of these three-momenta are shown. The distribution is focused clearly below 100M eV . On the right in figure 5.29(b) the correlation of the φ-components are shown. As expected the distribution is concentrated on the bisectioning line. The clear horizontal gaps in the acceptance of the Recoil Detector caused by the holding frame of the SSD is visible. Additional the quadrant 0 < p′RD /rad < π/2 for the RD has lower counts due to higer φ noise in the SSD. The distribution of the difference of the φ-component of these three- momenta is shown in figure 5.30. The black solid line is a Gaussian fit within the range of ±0.1rad and gives σ = 0.094rad. The red part is the 3σ range, which is used to improve exclusive event selection. Figure 5.31 shows the distribution of the missing invariant mass Mx 2 which is normalized for better comparison. For the black curve all general CHAPTER 5. PERFORMANCE 103 2 event selections for DVCS events except for Mx are performed. The usual 2 event selection on the missing invariant mass Mx for exclusive events is from −2.25GeV 2 to +2.89GeV 2 . Requesting a single proton track above the solid line in figure 5.28(b) results to the blue curve in figure 5.31(a). For the red curve the additional requirement is to have a coincidence for the missing proton between HERMES Forward Spectrometer and Recoil Detector within 3σ for the p′ distribution as shown in figure 5.30. The φ blue curve in figure 5.31(b) results if events with only a pion in the Recoil Detector are requested. Even without the final alignments and calibrations a clear improvement by the Recoil Detector is visible as the contamination is reduced. In future, with an improved tracking and particle identification much better results are expected. 5.4.8 Conclusion and Outlook The recently released HRC-production 06d and the DST-production 06d0 are used to discuss the current state. The distribution of usual observables has the expected shape and are in good agreement with previous produc- tions. The positron data investigated with the Recoil Detector give expected results too. Nevertheless some challenging tasks are still waiting for investigations. Some previous studies show that there is a slight difference in the response of the electromagnetic calorimeter for electrons and positrons. Beside this the final calibration and alignment of all detector parts are performed currently. The positron data gives a possibility to tune the tracking system and particle identification system of the Recoil Detector for the electron data as only data taken with the SFT are available yet. A lot of new informations about the response of the upgraded HERMES detector can be gathered by the study of the data with the Recoil Detector. By iterating of previous analysis with these new informations improvements will be possible to review the map of the constituents of the nucleon spin budget. CHAPTER 5. PERFORMANCE 104 × energy deposition [p.e.] 500 120 100 400 80 300 60 200 40 100 20 0 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 momentum [GeV] (a) 45000 energy deposition [p.e.] 500 40000 35000 400 30000 300 25000 20000 200 15000 10000 100 5000 0 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 momentum [GeV] (b) Figure 5.28: Correlation between energy deposition (in units of photo elec- trons) inside the SFT and the reconstructed momentum of the track (in GeV ). The distributions shows a separation between lower left pions and protons with requirements of two space points in the SFT (a) and with the requirements of four space points in SFT and SSD together (b). The solid line indicates a possible separation by dE ∗ p = 29. CHAPTER 5. PERFORMANCE 105 0.5 p'y -p'y [GeV] 0.4 10 RD 0.3 8 FS 0.2 0.1 6 -0 -0.1 4 -0.2 -0.3 2 -0.4 -0.5 0 -0.5 -0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4 0.5 p'FS-p'RD [GeV] x x (a) 3 p'RD [rad] 14 2 φ 12 1 10 8 0 6 -1 4 -2 2 -3 0 -3 -2 -1 0 1 2 3 p'FS φ [rad] (b) Figure 5.29: Correlation between several components of the three-momenta p′F S , p′RD of the recoiled proton, which are calculated by the missing in- variant mass technique using the HERMES FS and by measurement with the RD. (a) Correlation for the differences in the x-components versus the differences in the y-components. (b) Correlation between the φ-components. The horizontal gaps due to the holding frame of the SSD and the additional lower counts for the quadrant 0 < pRD /rad < π/2 due to higer noise in the φ SSD is clearly visible. CHAPTER 5. PERFORMANCE 106 180 counts [1] 160 140 120 100 80 60 40 20 0 -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 p'FS-p'RD φ φ [rad] (a) Figure 5.30: Distribution of the p′ difference calculated with data available φ from FS and with data measured with RD. A fit with an Gaussian function (solid black line) in the range ±0.1rad results to σ = 0.094rad, whereas a 3σ range (red) is used to improve exclusive event selection. CHAPTER 5. PERFORMANCE 107 normalized counts [1] 1 0.8 0.6 0.4 0.2 0 -5 0 5 10 15 20 25 30 35 missing invariant mass [GeV^2] (a) normalized counts [1] 1 0.8 0.6 0.4 0.2 0 -5 0 5 10 15 20 25 30 35 missing invariant mass [GeV^2] (b) Figure 5.31: Arbitrary normalized missing invariant mass distribution mea- sured with the HERMES Forward Spectrometer only (black) and with the contribution by the Recoil Detector on event selection for DVCS candidates. (a) Additional request of a single proton track (blue) and furthermore a coin- cidence for the missing proton between HERMES Forward Spectrometer and Recoil Detector within 3σ of the p′F S − p′RD distribution. (b) Additional φ φ request of a single pion track (blue). Chapter 6 Summary The standard model of particle physics is available to describe the the atomic world. Nevertheless the constituents of atoms like electrons, protons and neutrons are not completely understood. While the electrons seems to have point like properties the protons and the neutrons behave like compounded objects. The current standard model to describe protons and neutrons is based on quarks as partons, whereas photons, gluons, W- and Z-bosons are the carrier of the electromagnetic, the strong and the weak forces. In this model the aspect of gravity is not yet taken into account. Furthermore on large distance scale the calculations are not solved analytical yet. One par- ticular problem is the incomplete nucleon spin budget as well. Measurements at EMC (European Muon Collaboration) showed that the naive picture that three quarks contribute to the nucleon spin budget is not matching. The GPDs (Generalized Parton Distributions) illustrated in figure 6.1(a) are a recently developed model to describe transverse spatial distributions and longitudinal momentum distributions simultaneously without violating the uncertainty relation. The transverse spatial distributions named FFs (Form Factors) can be measured by elastic scattering, while the longitudinal momentum distributions named PDFs (Parton Distribution Functions) are measurable by deep inelastic scattering. GPDs are not only a combination of these two other models, rather new informations getting available. For example the sum rule researched by Ji gives the opportunity to access the nucleon spin budget. These GPDs can be measured with hard exclusive reactions where a scattered projectile interacts via a virtual photon. As an hard exclusive reaction the target stays after interaction intact and all produced particles are detected. Furthermore the GPDs can be measured selectively by the requested final state. For example ρ0 meson production enables the access to distributions of quarks and of gluons of the same and of linear order. But the cleanest way to access GPDs is DVCS (deeply virtual Compton scattering) as shown in figure 6.1(b). Due to same initial and final state the 108 CHAPTER 6. SUMMARY 109 k' (x+ξ) p (x-ξ) p k e e' e e' γ* γ γ q q' γ GPD(x,ξ,t) γ* γ* p t p' p p' p p' p p' (a) (b) (c) Figure 6.1: (a) Description of GPDs by average longitudinal momentum ¯ fraction x, fractional longitudinal momentum transfer ξ named skewness and momentum transfer t. (b) Principe of deeply virtual Compton scattering (c) Indistinguishable initial and final state of the Bethe-Heitler process. elastic BH (Bethe-Heitler) process is indistinguishable (see figure 6.1(c)). In order to calculate the cross section the amplitude of the photo production by BH τBH and by DVCS τDV CS are added together to the combined amplitude τ by (6.1) |τ |2 = |τBH |2 + |τDV CS |2 + τBH τDV CS + τDV CS τBH . ∗ ∗ For the kinematic range at the HERMES experiment the amplitude of the BH process is much larger than for DVCS by more than one order. As the pure BH contribution can be precisely calculated in the frame of quantum electro dynamics and by neglecting in leading order the pure DVCS contribution the mixed interference term gives the opportunity to access GPDs. One of the experiments carried out to make more investigations on the nucleon spin budget is the HERMES (HERA measurements of spin) exper- iment at HERA (Hadronen Elektronen Ring Anlage) at DESY (Deutsches Elektronen-Synchrotron) in Hamburg / Germany. The HERMES forward spectrometer as shown in figure 6.2 was designed to access the nucleon spin budget by DIS (deep inelastic scattering). Due to better understanding and new models like GPDs in the meantime the scientific program of HERMES was widely extended by investigations of different polarized gas targets. The HERMES detector consists of a tracking system to measure the momentum of charged particles which are deflected in the magnetic field of the HERMES magnet. A set of detectors provide a high efficient particle identification in order to reconstruct scattering processes. Even the recoiled particle can not be detected at HERMES due to the polarized target fa- cility exclusive reactions can be still investigated by the missing invariant 2 mass technique Mx which is illustrated in figure 6.3. Due to the limitation CHAPTER 6. SUMMARY 110 Figure 6.2: Schematic of the HERMES forward spectrometer. The tracking system is colored in red, while the particle identification system is in green. Dashed lines indicate acceptance range. of the resolution by the HERMES forward spectrometer the background contamination is around 15%. Figure 6.3: Missing invariant mass distributions for different reactions studied with Monte Carlo simulations. Exclusive region indicated by vertical solid lines is in the range of −2.25GeV 2 and +2.89GeV 2 . The contamina- tion is around 15%. In order to improve exclusivity and the resolution of kinematic variables, the collaboration decided to upgrade the HERMES forward spectrometer with the Recoil Detector. As this new part of the HERMES Detector sur- rounds the target area recoiling particles can be detected due to the big CHAPTER 6. SUMMARY 111 acceptance. This Recoil Detector consists of three sub detectors as shown in figure 6.4. A SSD (Silicon Strip Detector) which stays in the beam vacuum of HERA is the inner part. In the middle part the SFT (Scintillating Fiber Tracker) are positioned. The PD (Photon Detector) complete as the outer part. The aim of these components is to provide space points for the Recoil Detector internal tracking system which works independent of the HERMES tracking system and to provide particle identification by energy deposition. The Recoil Detector is enveloped by a 1T superconducting magnet for mo- mentum reconstruction. Iron Shielding Cryostat SC Coils SciFi Connector Plate Photon C3 Collimator Detector SciFi Si Detector Detector Cooling Silicon Si Detector Detector Connectors Target Cell Hybrid Flange 0.1 0.2 0.3 m Figure 6.4: The realization of the Recoil Detector. The design requirements were studied by extensive Monte Carlo simu- lations. The scattering polar angle θ versus the momentum of the recoiling proton p of several involved reactions together with the coverage by the Re- coil Detector are illustrated in figure 6.5. By identifying and suppression of background the remaining contamination can be decreased below 1%. The contributions of the Giessen group is the SFT which is one of the key part of the Recoil Detector. This subdetector was designed to contribute for particle identification and for track reconstruction. A set of tests were carried out to proof the designed properties and to settle down final configuration parameters. An alignment test run were per- formed to measure the SFT internal alignment of each single fiber over the CHAPTER 6. SUMMARY 112 1.4 1.4 p [GeV/c] a DVCS SciFi b BH/DVCS 1.2 1 0.8 0.6 Silicon 0.4 0.2 0 1.4 0 1.4 cρ d∆ 1.2 1 0.8 0.6 0.4 0.2 0 0 0 0.5 1 0 0.5 1 θ [rad] (a) (b) Figure 6.5: (a) Distributions of particle momentums p versus polar angle θ studied with MC simulations for (a) DVCS process, (b) DVCS / BH in- terference term, (c) ρ-meson production and (d) ∆ resonances or associated BH-process. The red area of low momentum protons are covered by the SSD, while the blue area of higher momentum protons are capped by the SFT. (b) The expected total remaining background investigated by Monte Carlo study is below 1%. whole active area. The database of these fibers consists of a list of poly- nominals up to fourth degree which describes each fiber path. A cosmic ray test run was set up to check the complete Recoil Detector and to implement the readout of the Recoil Detector to the HERMES data acquisition. The Recoil Detector was installed during the winter shutdown of HERA in 2005 and was taking data until the end of HERA in June 2007. The recently available 06d/06d0 data production was used for investi- gations of the current performance. The distributions of usual kinematic variables are in good agreement with analysis of previous productions. Investigations of elastic events show a very clear correlation for the re- constructed azimuthal angle between the HERMES forward spectrometer and the Recoil Detector as shown in figure 6.6. Preliminary calculations of the leading amplitude for beam charge asym- metry and for beam spin asymmetry versus the angle φ between the scatter- ing plane and the production plane was performed and shown in figure 6.7. Comparisons to previous results based on the 06b2 production show good agreement. The amplitudes for the beam charge asymmetry follows even with bigger error bars the shape of previous results while the results and error bars for beam spin asymmetry are closer. Deviations due to change in the HERMES coordinate system and differences in the response of the elec- CHAPTER 6. SUMMARY 113 azimuthal angle RD [rad] htemp counts [1] 3000 Entries 159367 Mean 0.1033 30000 RMS 0.8221 6 2500 5 25000 2000 4 20000 1500 3 15000 1000 2 10000 1 500 5000 0 0 0 0 1 2 3 4 5 6 -6 -4 -2 0 2 4 6 azimuthal angle FS [rad] azimuthal angle RD - azimuthal angle FS [rad] (a) (b) Figure 6.6: Correlation between HERMES forward spectrometer (FS) and the Recoil Detector (RD). (a) 2D-plot of adjusted azimuthal angle of the starting vertex. (b) Distribution of the adjusted differences. The small peak is as small as the fiber diameter of the SFT. tromagnetic calorimeter for electrons and protons can led to small deviations which are currently under investigations. ALU(φ) AC(|φ|) 0.2 0.05 0.1 -0 0 -0.05 -0.1 -0.1 -0.2 -0.15 -0.3 -0.2 -3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 φ [rad] |φ| [rad] (a) (b) Figure 6.7: (a) φ dependency of the leading amplitude for beam spin asym- metry. (b) |φ| dependency of the leading amplitude for beam charge asym- metry. The preliminary particle identification by the SFT as shown in figure 6.8 is already working and promising to be a good tool in PID scheme. The CHAPTER 6. SUMMARY 114 separation between pions in the lower left corner and the protons by the solid line is illustrated. × 45000 energy deposition [p.e.] energy deposition [p.e.] 500 120 500 40000 100 35000 400 400 30000 80 300 300 25000 60 20000 200 200 15000 40 10000 100 100 20 5000 0 0 0 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 momentum [GeV] momentum [GeV] (a) (b) Figure 6.8: Particle identification by combining energy deposition dE (in units of photon electrons) with reconstructed particle momentum p (in GeV). Distributions in case of two space points in SFT (a) and four space points in RD (b). The solid line indicates a possible separation by dE ∗ p = 29. 2 The normalized distribution of the missing invariant mass Mx in figure 6.9 shows the impact of the Recoil Detector for DVCS event selection. As reminder the reaction of DVCS events can be expressed by (6.2) l + p → l ′ + γ + p′ where l and l′ are the initial and scattered lepton, γ is the generated real photon and p and p′ are the initial and scattered proton. The black line shown the distribution after applying all event selections for DVCS events 2 but the one for the missing invariant mass Mx . The usual event selection on Mx 2 for exclusive events is from −2.25GeV 2 to +2.89GeV 2 . By requesting an additional event selection for a single proton track above the solid line in figure 6.8(b) in the Recoil Detector yields to the red curve in figure 6.9(a). Due to undetected bremsstrahlung a small tail to higher missing invariant mass remains. The blue curve in figure 6.9(b) results if events with only a pion in the Recoil Detector are requested. Even without final alignments and calibrations a clear improvements for background suppression is shown. Currently the fine tuning of the configuration parameters like alignment, calibration and particle identification is in progress and a main data pro- duction will be started soon. Informations gathered by investigations with CHAPTER 6. SUMMARY 115 normalized counts [1] normalized counts [1] 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 -5 0 5 10 15 20 25 30 35 -5 0 5 10 15 20 25 30 35 missing invariant mass [GeV^2] missing invariant mass [GeV^2] (a) (b) Figure 6.9: Arbitrary normalized missing invariant mass distribution mea- sured with the HERMES Forward Spectrometer only (black) and with the contribution by the Recoil Detector on event selection for DVCS candidates. (a) Additional request of a single proton track (blue) and furthermore a coin- cidence for the missing proton between HERMES Forward Spectrometer and Recoil Detector within 3σ for the p′F S − p′RD distribution. (b) Additional φ φ request of a single pion track (blue). the Recoil Detector will be used to iterate previous analysis and to improve the understanding of the nucleon spin budget. Appendix A Data formats This chapter describes the data format of the standalone readout and of the experiment readout whereas a '*' denotes a variable content. A.1 Data format for standalone readout This chapter describes the data format which was used for the standalone readout. In general the data stream is organized as a stream of nodes. Each node consists of a head part to identified the purpose and a data part for the raw data. In the following sections the structure of the all parts are presented. A.1.1 Node structure Each node (see table A.1) starts with four signature bytes in order to identify the content of the node. A 32-bit unsigned integer is used to indicate the amount of data bytes containing this node without the head part itself. After this head part the data part of the according size and content are stored. This yields to a total length in bytes of 8+len. byte byte content comment offset size 0 1 * signature 1 1 1 * signature 2 2 1 * signature 3 3 1 * signature 4 4 4 len length of node data in bytes 8 len * data part of the node Table A.1: Node structure 116 APPENDIX A. DATA FORMATS 117 A.1.2 Sequence of nodes The stream of data (see table A.2) consists of a sequence of nodes. The sequence starts with a signature to indicate the content of the data stream. A head initiator and a head terminator surrounds the head part. Each event starts with a data initiator and ends with a data terminator. In between a node for each requested sub detector can be found in any order. The list of sub detectors are the Silicon Strip Detector, the ADC part of the Scintillating fiber tracker, the TDC part of the Scintillating fiber tracker and the Photon Detector. The data stream ends with a tail initiator and a tail terminator. type comment signature a signature entry to identify the stream content head initiator a node to initiate the head content head terminator a node to terminate head part data initiator a node to initiate the data of a event detector data readout dependent list of nodes for the sub detector data terminator the terminator for each data initiator tail initiator a node to initiate the tail content tail terminator a node to terminate tail part Table A.2: Sequence of nodes of the data stream. Signature This signature (see table A.3) is used like the linux 'file' command in order to identify the content of the stream and has a length of 8 bytes. byte byte content comment offset size 0 1 4 byte size of length variable in this case 4 times 8 bits = 32 bits unsigned integer 1 1 ';' first filler character 2 1 ')' second filler character 3 1 0 c-string like terminator 4 4 0 zero length for no remaining data Table A.3: Content of the signature node. APPENDIX A. DATA FORMATS 118 Head initiator The head initiator (see table A.4) contains the status information (see table A.5) about the run number and the timestamp of the start of data taking. byte byte content comment offset size 0 1 'H' signature 1 1 1 'E' signature 2 2 1 'A' signature 3 3 1 'D' signature 4 4 4 16 status informations Table A.4: Head initiator content byte byte content comment offset size 0 4 0 versioning tag 4 4 * run number 8 8 * start of data taking timestamp of struct timeval filled by the call of the c-function 'gettimeofday' Table A.5: Status informations of the head initiator Head terminator The head terminator (see table A.6) does not contain any additional infor- mation yet. byte byte content comment offset size 0 1 'h' signature 1 1 1 'e' signature 2 2 1 'a' signature 3 3 1 'd' signature 4 4 4 0 zero length for no remaining data Table A.6: Head terminator content APPENDIX A. DATA FORMATS 119 Data initiator The data initiator (see table A.7) contains the status informations (see table A.8) about the event number within this run, the timestamp of the event and the source of the trigger. byte byte content comment offset size 0 1 'D' signature 1 1 1 'A' signature 2 2 1 'T' signature 3 3 1 'A' signature 4 4 4 20 status informations Table A.7: Data initiator content byte byte content comment offset size 0 4 0 versioning tag 4 4 * event number 8 8 * timestamp of the event of struct timeval filled by the call of the c-function 'gettimeofday' 16 4 * source of trigger Table A.8: Status informations of the data initiator Data node for the Silicon Strip Detector The data node (see table A.9) contains the data of the Silicon Strip Detec- tor (see section A.2.1). This node is only available if this sub detector is requested to be read out during the data taking. byte byte content comment offset size 0 1 'S' signature 1 1 1 'I' signature 2 2 1 '' signature 3 3 1 '' signature 4 4 4 len remaining data of the event Table A.9: Data node for the Silicon Strip Detector APPENDIX A. DATA FORMATS 120 Data node for the ADC of the Scintillating Fiber Tracker The data node (see table A.10) contains the ADC data of the Scintillat- ing Fiber Tracker which consists of several continuous readout controller buffers (see section A.2.2). This node is only available if this sub detector is requested to be read out during the data taking. byte byte content comment offset size 0 1 'S' signature 1 1 1 'F' signature 2 2 1 'f' signature 3 3 1 'i' signature 4 4 4 len remaining data of the event Table A.10: Data node for the ADC part of the Scintillating Fiber Tracker Data node for the TDC of the Scintillating Fiber Tracker For the first implementation of the TDC readout a different TDC named v767 [CAE08c] from CAEN was used. Therefore the data structure of the standalone readout of the TDC chain has the buffer structure of the v767. Even as the TDC hardware was changed to the v1190A [CAE08b] from CAEN the data structure was kept for compatibility purpose. The readout was adjusted to mimicry the buffer structure accordingly. The data node (see table A.11) contains the TDC data of the Scintillat- ing Fiber Tracker which consists of a (see table A.12). Detailed technical informations can be found in [CAE08c]. This node is only available if this sub detector is requested to be read out during the data taking. byte byte content comment offset size 0 1 'S' signature 1 1 1 'F' signature 2 2 1 'd' signature 3 3 1 'y' signature 4 4 4 len remaining data of the event Table A.11: Data node for the TDC part of the Scintillating Fiber Tracker APPENDIX A. DATA FORMATS 121 byte byte comment offset size 0 4 header informations 4 len hit data 8+len 4 end-of-block informations Table A.12: Buffer structure of the TDC v767 from CAEN. Data node for the Photon Detector The data node (see table A.13) contains the data of the Photon Detector (see section A.2.3). This node is only available if this sub detector is requested to be read out during the data taking. byte byte content comment offset size 0 1 'P' signature 1 1 1 'D' signature 2 2 1 '' signature 3 3 1 '' signature 4 4 4 len remaining data of the event Table A.13: Data node for the Photon Detector Data terminator The data terminator (see table A.14) does not contain any additional infor- mation yet. byte byte content comment offset size 0 1 'd' signature 1 1 1 'a' signature 2 2 1 't' signature 3 3 1 'a' signature 4 4 4 0 zero length for no remaining data Table A.14: Data terminator content Tail initiator The tail initiator (see table A.15) does not contain any additional informa- tion yet. APPENDIX A. DATA FORMATS 122 byte byte content comment offset size 0 1 'T' signature 1 1 1 'A' signature 2 2 1 'I' signature 3 3 1 'L' signature 4 4 4 0 zero length for no remaining data Table A.15: Tail initiator content Tail terminator The tail terminator (see table A.16) contains the status (see tableA.17) informations about the run number, the timestamp for the end of data taking, the number of containing events and the back reference to be start of the tail entry terminator as the very last item for direct access of these data. byte byte content comment offset size 0 1 't' signature 1 1 1 'a' signature 2 2 1 'i' signature 3 3 1 'l' signature 4 4 4 24 status informations Table A.16: Tail terminator content byte byte content comment offset size 0 4 0 versioning tag 4 4 * run number 8 8 * end of data taking timestamp of struct timeval filled by the call of the c-function 'gettimeofday' 16 4 * number of stored events 20 4 * backreference to the start of the tail entry terminator for a direct short cut access Table A.17: Status informations of the tail terminator APPENDIX A. DATA FORMATS 123 A.2 Data format for experiment readout The data format for the experiment consists of the individual buffers for each Recoil subdetector and are event based as well. A.2.1 Silicon Strip Detector The readout of the Silicon Strip Detector consist on four HADC (HERMES Analog to Digital Converter) VME-modules which was originally designed for the Lambda Wheel Detector. The complete buffer is just the sequence of four such HADC modules where each of them contains a header entry (see table A.18), a trigger counter entry (see table A.19), an optional trailer entry (see table A.20) and optional data entry (see table A.21). Extended informations about the Silicon Strip Detector can be found in [Pic08] and in [Vil08]. first last bit content comment bit bit size 0 11 12 * event number 12 15 4 * module number 16 28 13 * length of events alias number of data entries 29 29 1 * indicator for trailer error 30 30 1 * extended header of trailer available 31 31 1 1 indicate header informations Table A.18: Header informations are packed in a 32-bit unsigned integer. first last bit content comment bit bit size 0 11 12 * number of accepted triggers 12 19 8 * number of rejected triggers 20 31 12 * number of recognized triggers Table A.19: Trigger counter informations are packed in a 32-bit unsigned integer. A.2.2 Scintillating Fiber Tracker In contrast to the standalone readout for the experiment readout by the DSP the buffers for the ADC and TDC are merged to a common block (see table A.22). The common block consists of the 32-bit length value of the TDC content, the TDC content itself, the 32-bit length value of the ADC APPENDIX A. DATA FORMATS 124 first last bit content comment bit bit size 0 7 8 * ADC0 trailer 8 15 8 * ADC1 trailer 16 24 8 * ADC2 trailer 25 31 8 * ADC3 trailer Table A.20: Trailer entry consists of four values consisting of a 32-bit unsigned integer. first last bit content comment bit bit size 0 9 10 * raw ADC value 10 18 9 * raw common mode value 19 25 7 * channel number 26 28 3 * Helix number 29 30 2 * ADC chip number 31 31 1 0 indicate data informations Table A.21: Each hit data is packed in a 32-bit unsigned integer. content, the ADC value itself and a status buffer for additional informations about the ADC readout. The structure of the TDC data follows the original v1190A [CAE08b] buffer structure in contrast to the standalone readout. The buffer for the ADC can contain several continuous readout controller buffers (see section A.2.2). The content of the status information buffer is summarized in section A.2.2. byte byte comment offset size 0 4 length of TDC buffer in bytes inclusive this 4 len TDC TDC v1190 from CAEN buffer structure 4 4 length of ADC block in bytes inclusive this +len TDC 8 len ADC several readout controller buffers +len TDC 12 len DBG status information buffer +len TDC +len ADC Table A.22: Data buffer for the Scintillating Fiber Tracker for the experi- ment readout. APPENDIX A. DATA FORMATS 125 Readout controller buffer structure Each of the readout controller buffer consists of a header part which is always available and a hit data part where the amount of data depends on sparsification. The entries of such a buffer are all 32-bit integer values and has the sequence which is shown in table A.23 whereas each hit data has the localization information and the adc value together (see table A.24). A remark to reduce confusion about the content of the adc values. Due to hardware implementation the pedestal has a high raw adc value and higher signals has lower raw adc values. byte byte content offset size 0 4 event size in bytes inclusive the this 4 4 event decoder ( not used ) 8 4 event ID for error indication 12 4 event tagging for synchronization check 16 len len of remaining hit data of the event Table A.23: Buffer structure of the readout controller. The amount of hit data depends on sparsification. N.B. due to hardware implementation the pedestal has a high raw adc value and higher signals has lower raw adc values. first last bit comment bit bit size 0 9 10 raw adc value 10 15 6 channel number 16 18 3 module number 19 21 3 port number 22 25 4 readout controller number 26 31 6 not used Table A.24: Hit data content packed in a 32-bit unsigned integer. Status information buffer For the readout of the SFT additional status informations are saved for offline data quality checks. It consists of 3 times 32-bit integers for the DTU and 3 times 32-bit integers for each readout controller. The content for the DTU covers low level informations as bit pattern. DTU0 contains informations about status bits, control bits and version in- formations. DTU1 covers four counters to track the internal synchronization APPENDIX A. DATA FORMATS 126 by a bank switch counter, a readout counter, a trigger counter and a tagging counter. DTU2 stores internal state and transition informations of the state machine inside the FPGA which is mounted on the DTU. The monitoring content of the readout controller has informations which are comparable to the DTU. RCdbg0 contains informations about electronic state of certain signals of the handshaking between DTU and RC. RCdbg1 reflect the tagging counter and the trigger counter. RCdbg2 covers the counter for the readout and for the bank switching. An extended documentation on bit level can be found in [SFT+06]. All these saved informations can be used to verify the data integrity of the SFT and a tools is available to detect desynchronizations and to resynchronized them again. A.2.3 Photon Detector The readout of the Photon Detector consist of six commercial charge to digital converter V792 from CAEN. The complete buffer is just the sequence of six such V792 buffers where each of them contains a header entry (see table A.25), hit data entry (see table A.26) and a end-of-block entry (see table A.27). The amount of hit data depends on sparsification. Further informations about the Photon Detector are available in [Haa07] and in [CAE08a]. first last bit content comment bit bit size 0 7 8 * not defined 8 13 6 * number of data entries 14 15 2 0 not defined 16 23 8 * crate number 24 26 3 010 bit pattern for header content 27 31 5 * slot number Table A.25: Header information of the V792 buffer. APPENDIX A. DATA FORMATS 127 first last bit content comment bit bit size 0 11 12 * raw adc value 12 12 1 * overflow bit 13 13 1 * underflow bit 14 15 2 * not defined 16 20 5 * channel number 21 23 3 * not defined 24 26 3 000 bit pattern for data content 27 31 5 * slot number Table A.26: Hit data informations of the V792 buffer first last bit content comment bit bit size 0 23 24 * event counter 24 26 3 100 bit pattern for end-of-block content 27 31 5 * slot number Table A.27: End-of-block informations of the V792 buffer Bibliography [Ack+98] The HERMES Spectrometer K. Ackerstaff, et al., Nucl. Instrum. Meth. A417 (1998) 230 [ADA95] Entity-Relationship Programming System, Version 3.3 Program- ming Techniques Group, ECP Division, CERN (1995), http://adamo.web.cern.ch/Adamo/ADAMO ENTRY.html [AD08a] Single Supply, Rail to Rail Low Power FET-Input Op Amp AD820 Analog Devices Inc. (former Burr Brown) [AD08b] ADSP-21061L SHARC Analog Devices Inc. [Air+01] Measurement of the Beam-Spin Azimuthal Asymmetry Associated with Deeply-Virtual Compton Scattering A. Airapetian, et al., Phys. Rev. Lett. 87 (2001) 182001 [Air+05a] The Time-of-Flight Technique for the HERMES experiment A. Airapetian, et al., Nucl. Instr. Meth. A540 (2005) 305 [Air+05b] The HERMES polarized hydrogen and deuterium gas target in the HERA electron storage ring A. Airapetian, et al., Nucl. Instr. Meth. A540 (2005) 68 [Air+06] The Beam-Charge Azimuthal Asymmetry and Deeply Virtual Compton Scattering A. Airapetian, et al, arXiv:hep-ex/0605108v3 (2006) [Air+08] Measurement of Azimuthal Asymmetries With Respect To Both Beam Charge and Transverse Target Polarization in Exclusive Electro- production of Real Photons A. Airapetian, et al., arXiv:hep-ex/0802.2499v2 (2008) ˇ [Ako+02] The HERMES Dual-Radiator Ring Imaging Cherenkov Detector N. Akopov, et al., Nucl. Instrum. Meth. A479 (2002) 511 [And+01] Multiwire Proportional Chambers in the HERMES experiment A. Andreev, et al., Nucl. Instrum. Meth. A479 (2001) 482 128 BIBLIOGRAPHY 129 [Asc+00] Optical characterization of n=1.03 silica areogel used as radiator in the RICH of HERMES E.C. Aschenauer, et al., Nucl. Instrum. Meth. A440 (2000) 338 [Ash+88] A measurement of the spin asymmetry and determination of the structure function g1 in deep inelastic scattering J. Ashman, Phys. Lett. B206 (1988) 364 [Ash+89] An investigation of the spin structure of the proton in deep in- elastic scattering of polarized muons on polarized protons J. Ashman, Phys. Lett. B328 (1989) 1 [Ava+98] Performance of the Electromagnetic Calorimeter of the HERMES Experiment H. Avakian, et al., Nucl. Instrum. Meth. A417 (1998) 69 [Bac+04] Single-spin asymmetries: the Trento conventions A. Baccetta, U. D'Alesio, M. Diehl, C.A. Miller, arXiv:hep- ph/0410050v2 (2004) [Bar+93] The HERA polarimeter and the first observation of electron spin polarization at HERA D.P. Barber, et al., Nucl. Instr. Meth. A329 (1993) 79 [Bau+02] An Atomic Beam Polarimeter to Measure the Nuclear Polarisa- tion in the HERMES Gaseous Polarized Hydrogen and Deuterium Target C. Baumgarten, et al., Nucl. Instr. Meth. A482 (2002) 606 [Bau+03a] Measurements of atomic recombination in the HERMES polar- ized hydrogen and deuterium storage cell target C. Baumgarten, et al., Nucl. Instr. Meth. A496 (2003) 263 [Bau+03b] The storage cell of the polarized internal H/D gas target of the HERMES experiment at HERA C. Baumgarten, et al., Nucl. Instr. Meth. A496 (2003) 277 [Bau+03c] A gas analyzer for the internal polarized target of the HERMES experiment C. Baumgarten, et al., Nucl. Instr. Meth. A508 (2003) 268 [Bec+02] The Longitudinal Polarimeter at HERA M. Beckmann, et al., Nucl. Instr. Meth. A479 (2002) 334 [Beh+07] Test Beams at DESY T. Behnke, et al., note, http://www.eudet.org/e26/e28/e182/e283/eudet-memo-2007-11.pdf [Bel+02] Theory of Deeply Virtual Compton Scattering on the nucleon A. Belitsky, et al., Nucl. Phys. B629 (2002) 323 BIBLIOGRAPHY 130 [Ben+01] The luminosity monitor of the HERMES experiment at DESY T. Benisch, et al., Nucl. Instr. Meth. A471 (2001) 314 [Ben98] Polarisierte Bhabha-Streuung und Luminositaetsmessung im HERMES-Experiment T. Benisch, Dissertation, HERMES 98-045 (1998) [Ber+95] Design and Performance of the Large HERMES Drift Chambers S. Bernreuther, et al., Nucl. Instrum. Meth. A367 (1995) 96 [Ber+98] The HERMES Back Drift Chambers S. Bernreuther, et al., Nucl. Instrum. Meth. A416 (1998) 45 [Bjo69] Asymptotic Sum Rules at Infinite Momentum J.D. Bjoerken, Phys. Rev. 179 (1969) 1547 u ˇ [Boe00] Das Auslesesystem f¨r den Ringabbildenden Cherenkovdetektor im HADES Spektrometer u M. Boehmer, Diploma Thesis, TU M¨nchen 2000 [Boh13] On the Constitution of Atoms and Molecule N. Bohr, Philosophical Magazine, Series 6, Volume 26 (1913) 1-25, 476- 502 and 857-875. [Bra+01] The HERMES forward tracking chambers: construction, operation and aging effects J.T. Brack, et al., Nucl. Instr. Meth. A469 (2001) 47 [Bro+07] Moments of generalized parton distributions and quark angular momentum of the nucleon o D. Br¨mmel, et al., arXiv:hep-lat/0710.1534v1 (2007) [Buo+86] HERA variable-energy "mini" spin rotator and head-on ep colli- sion scheme with choice of electron helicity J .Buon, K. Steffen, Nucl. Instr. Meth. A245 (1986) 248 [CAE08a] CAEN VME V792 (32 Channel Multievent QDC) CAEN S.p.A. - PI 00864500467 http://www.caen.it/nuclear/product.php?mod=V792&fam=vme&fun=qdc [CAE08b] CAEN VME V1190A (128 Channel Multihit Multievent TDC) CAEN S.p.A. - PI 00864500467 http://www.caen.it/nuclear/product.php?mod=V1190A&fam=vme&fun=tdc [CAE08c] CAEN VME V767 (128 Channel Multihit Multievent TDC) CAEN S.p.A. - PI 00864500467 http://www.caen.it/nuclear/product.php?mod=V767&fam=vme&fun=tdc BIBLIOGRAPHY 131 [Dem07] The polarization of Lambda hyperons in quasi-real photoproduction M. Demey, Dissertation, HERMES 07-005 (2007) [Dia+02] Design and commissioning of the GSI pion beam J .Diaz, et al., Nucl. Instr. Meth. A478 (2002) 511 [Die03] Generalized Parton Distributions M. Diehl, arXiv:hep-ph/0307382v2 (2003) [Die+04a] Generalized parton distributions from nucleon form factor data M. Diehl, et al., arXiv:hep-ph/0408173v2 (2004) [Die+04b] Quarks vs. gluons in exclusive rho electroproduction M. Diehl, A.V. Vinnikov, arXiv:hep-ph/0412162v2 (2004) [Due+92] Test of a polarized hydrogen gas target based on the storage cell technique u M. D¨ren, et al., Nucl. Instr. Meth. A322 (1992) 13 [Due95] The HERMES Experiment: From the Design to the First Results u M. D¨ren, Habilitation, HERMES 95-002 (1995) [Eid+04] Passage of particles through matter S. Eidelmann, et al., Phys. Lett. B592 (2004) 1 [Ela96] EL4421C/22C/41C/42C/43C/44C-data sheet Rec.C. Elantec inc. (1996) [Ell03] Beam-Charge and Beam-Spin Azimuthal Asymmetries in Deeply- Virtual Compton Scattering F. Ellinghaus, Dissertation, HERMES 03-044 (2003) [Ell+05] Can the total angular momentum of u-quarks in the nucleon be ac- cessed at HERMES? F. Ellinghaus, W.-D. Nowak, A.V. Vinnikov, Z. Ye, arXiv:hep- ph/0506264v1 (2005) [Ell07] DVCS at HERMES: Recent Results F. Ellinghaus, conference proceedings, arXiv:hep-ex/0710.5768v1 (2007) [Eme96] Modeling of the HERMES Transition Radiation Detector J. V. Emerson, M. Sc. Thesis, HERMES 96-038 (1996) [GEA93] GEANT - Detector Description and Simulation Tool CERN Program Library Long Writeup W5013 (1993) [Goe+01] Hard Exclusive Reactions and the Structure of Hadrons K. Goeke, M.V. Polyakov, M. Vanderhaegen, arXiv:hep-ph/0106012v2 (2001) BIBLIOGRAPHY 132 [Gre+04] Study of the First Prototype for the HERMES Silicon Recoil De- tector with the ZEUS Beam Telescope I.M. Gregor, note, HERMES 04-016 (2004) [Gri87] Introduction to Elementary Particles D.J. Griffiths, Wiley, John & Sons, Inc., ISBN 0-471-60386-4 (1987) [Guz+06] The dual parameterization of the proton generalized parton dis- tribution functions H and E and description of the DVCS cross sections and asymmetries V. Guzey, T. Teckentrup, arXiv:hep-ph/0607099 (2006) [Haa07] The Hermes recoil photon detector and nuclear pt -broadening at Hermes Y. van Haarlem, Dissertation, HERMES 07-020 (2007) [Hae+07] Nucleon Generalized Parton Distributions from Full Lattice QCD a P. H¨gler, et al., arXiv:hep-lat/0705.4295v2 [Ham00] Multi-anode Photomultiplier Tube Assembly H7546 data sheet Hamamatsu (2000) [Har04] The TDC readout of the 12th dynode for the SciFi PMTs M. Hartig, note, HERMES 04-066 (2004) [Hee03] Quasi-real photo-production of hyperons on polarized 1,2 H targets D. Heesbeen, Dissertation, HERMES 03-052 (2003) [Her+03] HERMES DAQ, HERMES DAQ group, notes (2008), http://www-hermes.desy.de/groups/daqgrp [Her+08] Hermes DataCops HERMES Data Quality group, notes (2008) http://www-hermes.desy.de/groups/daqlgrp [Her+93] HERMES Technical Design Report HERMES collaboration, note, HERMES 93-015 (1993) HERMES collaboration, DESY-PRC-93-06 (1993) [Hoe+05] Performance of the Recoil Scintillating Fibre Tracker in a Test Beam at GSI M. Hoek, et al., note, HERMES 05-013 (2005) [Hoe06] Design and Construction of a Scintillating Fibre Tracker for mea- suring Hard Exclusive Reactions at HERMES M. Hoek, Dissertation, HERMES 06-098 (2006) BIBLIOGRAPHY 133 [Hoe+07] Performance and response of scintillating fiber modules to protons and pions M. Hoek, et al., Nucl. Instr. Meth. A572 (2007) 808 [Hri+05] HERMES Recoil Silicon Detector Calibration to MIPs at T22 at DESY I. Hristova, note, HERMES 05-014 (2005) [Jaf96] Spin, Twist and Hadron Structure in Deep Inelastic Processes R.L. Jaffe, arXiv:hep-ph/9602236v1 (1996) [Ji97a] Deeply Virtual Compton Scattering X.D. Ji, Phys. Rev. D55 (1997) 7114 [Ji97b] Gauge-invariant decomposition of nucleon spin X.D. Ji, Phys. Rev. Lett. 78 (1997) 610 [Kai+02] The HERMES Recoil Detector R. Kaiser, et al., note, HERMES 02-003 (2002) [Kai+97] Particle Identification at HERMES R. Kaiser, et al., note, HERMES 97-025 (1997) [Kas+99] Fast detector readout for the HADES-RICH u A. Kastenm¨ller, et al., Nucl. Instr. Meth. A433 (1999) 438 [Kiv+01] DVCS on the nucleon: Study of the twist - three effects N. Kivel, M. Polyakov, M. Vanderhaeghen, Phys. Rev. D63 (2001) 114014 [Kor+02] Future measurements of Deeply Virtual Compton Scattering V.A. Korotkov, W.D. Nowak, Eur. Phys. J. C23 (2002) 455 [Kra05] Deeply Virtual Compton Scattering and the HERMES-Recoil De- tector B. Krauss, Dissertation, HERMES 05-007 (2005) [Kro07] Hard exclusive scattering at Jlab P. Kroll, arXiv:hep-ph/0710.2771v1 (2007) [Maz+07] Deeply Virtual Compton Scattering off the neutron M. Mazouz, et al., arXiv:nucl-ex/0709.0450v2 (2007) [McL+93] EPIO Experimental Physics Input Output I. McLaren, et al., CERN Program Library entry I101 (1993) [Men01] Determination of the Gluon Polarization in the Nucleon F. Menden, Dissertation, HERMES 01-073 (2001) BIBLIOGRAPHY 134 [Mue+94] Wave functions, evolution equations and evolution kernels from light-ray operators of QCD D. Muller, et al., Fortschr. Phys. 42 (1994) 101 [Nas+03] The HERMES polarized atomic beam source A. Nass, et al., Nucl. Instr. Meth. A505 (2003) 633 [PDG08] The Review of Particle Physics W.M. Yao, et al. (Particle Data Group), J. Phys. G 33, 1 (2006) and 2007 partial update for the 2008 edition [Pic08] Commissioning of the Recoil Silicon Detector for the HERMES Ex- periment N. Pickert, Dissertation, HERMES 08-002 (2008) [Rei07] Quasi-real photo-production of hyperons and their impact on the Lambda polarization measurements A.J. Reischl, Dissertation, HERMES 07-004 (2007) [Ros+07] Transverse Target-Spin Asymmetry of Exclusive ρ0 Meson Pro- duction on Proton at HERMES A. Rostomyan, et al., arXiv:hep-ex/0707.2486v1 (2007) [Rub06] Search for the Pentaquark States in Lepton-Nucleon Scattering at HERMES ac L. Rub´ˇek, Dissertation, HERMES 06-071 (2006) [Ruh99] Measurement of the Polarized Quark Distributions of the Nucleon at HERMES M. Ruh, Dissertation, HERMES 99-051 (1999) [San+01] The Gassiplex0.7-2 Integrated Front-End Analog Processor for the HMPID and the Dimuon Spectrometer of ALICE J.C. Santiard and K. Marent [Alice Collaboration], CERN-Alice- PUB-2001-49 (2001) [San+94] GASPLEX: a low noise analog signal processor for readout of gaseous detectors J.C. Santiard, et al. CERN-ECP-94-17 (1994) [Sch+98] The HERMES Polarized 3 He Internal Gas Target D. De Schepper, et al., Nucl. Instr. Meth. A419 (1998) 16 [Sei04] The HERMES recoil detector: a combined silicon strip and scintil- lating fibre detector for tracking and particle identification B. Seitz, Nucl. Instr. Meth. A535 (2004) 538 BIBLIOGRAPHY 135 [SFT+06] Technical SciFi Documents HERMES Recoil Detector SFT group http://www-hermes.desy.de/groups/rcoilgrp/recoil/techdoc sft.php [Shi+98] A High Precision Laser Alignment Monitoring System for HERMES Tracking Detectors T.-A. Shibata, et al., Nucl. Instr. Meth. A411 (1998) 75 [Sok+63] On Polarization and Spin Effects in the Theory of Synchrotron Radiation A.A. Sokolov, I.M. Ternov, Dokl. Akad, Nauk SSSR 153 (1963) 1052 [Sok+64] On polarization and spin effects in synchrotron radiation theory A.A. Sokolov, I.M. Ternov, Sov. Phys. Doklady 8 (1964) 1203 [Som03] Identifikation von Pionen und Protonen mit einem Detektor aus szintillierenden Fasern W. Sommer, Diploma Thesis, HERMES 03-046 (2003) [Som15] Sitzungsber. Munchen A. Sommerfeld, Akad. d. Wiss., 425 (1915) 459 [Sta06] Superconducting Magnetic Systems for High Energy Polarized Physics M. Statera, Dissertation, HERMES 06-069 (2006) [Ste+03] Polarized Gas Targets E. Steffens, W. Haeberli, Rep. Prog. Phys. 66 (2003) 1887 [Ste+05] Alignment of the SciFi Detector at the DESY22 electron testbeam H. Stenzel, note, HERMES 05-063 (2005) [Ter+61] On possibility of polarization of an electron beam due to relativistic radiation in a magnetic field I.M. Ternov, Y.M. Loskutov and L.I. Korovina, Zhur. Eksp. Teor. Fis., 41 (1961) 1294 [Thi96] The Gas System for the HERMES Transition Radiation Detector D.M. Thiessen, M. Sc. Thesis, HERMES 96-028 (1996) [Van+99] Deeply virtual electroproduction of photons and mesons on the nucleon : leading order amplitudes and power corrections M. Vanderhaeghen, P.A.M. Guichon, M. Guidal, arXiv:hep- ph/9905372v1 (1999) [Vil08] Detection of Low Momentum Protons with the New HERMES Recoil Detector I. Vilardi, Dissertation, HERMES 08-003 (2008) BIBLIOGRAPHY 136 [Wan+95] DAD - Distributed Adamo Database system at HERMES W. Wander, et al., note, HERMES 95-040 (1995) [Wen01] Particle Identification for HERMES Run I J. Wendland, note, HERMES 01-067 (2001) [Wen99] Improved Particle Identification at HERMES and Polarised Va- lence Quark Distributions in the Proton J. Wendland, M. Sc. Thesis, HERMES 99-016 (1999) [Wii+79] PETRA Resultate u.a. ein 3-Jet Event und Hinweise auf Gluon Brems. (vorlaeufig) B. Wiik, conference Neutrino 79 Bergen, 18.-22.6.1979 [Xil] XC4000XLA/XV Field Programmable Gate Arrays Xilinx, Inc. [Yas07] SSD+SFT alignment first results S. Yaschenko, HERMES Technical Recoil meeting, 26.03.2007 [Ye07] Transverse Target-Spin Asymmetry Associated with Deeply Virtual Compton Scattering on the Proton and A Resulting Model-Dependent Constraint on the Total Angular Momentum of Quarks in the Nucleon Z. Ye, Dissertation, HERMES 07-002 (2007) List of Figures 2.1 Schematic of deep inelastic scattering . . . . . . . . . . . . . . 11 2.2 Definition of the GPDs parameter . . . . . . . . . . . . . . . 14 2.3 Diagram of exclusive reactions with scattering lepton . . . . . 15 2.4 Diagram for deeply virtual Compton scattering . . . . . . . . 16 2.5 Diagram of Bethe-Heitler process . . . . . . . . . . . . . . . . 17 2.6 Spatial illustration of DVCS reactions . . . . . . . . . . . . . 18 2.7 Cross section Comparison of DVCS and BH . . . . . . . . . . 20 2.8 Beam spin asymmetry . . . . . . . . . . . . . . . . . . . . . . 21 2.9 Beasm charge asymmetry . . . . . . . . . . . . . . . . . . . . 22 2.10 Momenta and azimuthal angles for DVCS reaction . . . . . . 23 2.11 Model-dependent constraints on total angular momentum . . 24 3.1 Photo of DESY with HERA and PETRA illustration . . . . . 26 3.2 Set of accelerators at DESY . . . . . . . . . . . . . . . . . . . 27 3.3 Illustration of HERA . . . . . . . . . . . . . . . . . . . . . . . 28 3.4 Self polarization of lepton beam . . . . . . . . . . . . . . . . . 29 3.5 Illustration of the gas target equipment . . . . . . . . . . . . 32 3.6 Illsutration of the HERMES Forward Spectrometer . . . . . . 33 3.7 Setup of the luminosity monitor . . . . . . . . . . . . . . . . . 33 3.8 Performance plot of the Lumnisosity monitor . . . . . . . . . 34 3.9 Illustration of the tacking system . . . . . . . . . . . . . . . . 36 3.10 Illustration of the HERMES trackfinding algorithm . . . . . . 39 3.11 Performance of the HERMES tracking system . . . . . . . . . 40 3.12 Setup of the upper half of the RICH . . . . . . . . . . . . . . 40 3.13 Reponse of RICH for different particles . . . . . . . . . . . . . 41 3.14 Setup of the upper half of the TRD . . . . . . . . . . . . . . . 42 3.15 Response of the TRD for different particles . . . . . . . . . . 43 3.16 Setup of the hodoscope H1/H2 and the el.-mag. calorimeter . 43 3.17 Response of the electromagnetic calorimeter . . . . . . . . . . 44 3.18 Setup of hodoscope H0 and resposne of hodoscope H2 . . . . 45 3.19 Performance of the particle identification system . . . . . . . 46 4.1 Illustration of the modification of the HERMES Detector . . 47 137 LIST OF FIGURES 138 4.2 MC Missing invariant mass distribution for different reactions 48 4.3 Properties for different scattering reactions . . . . . . . . . . 50 4.4 Overview of the Recoil Detector . . . . . . . . . . . . . . . . . 51 4.5 Illustration of the modified HERMES forward spectrometer . 51 4.6 Photo of the new target cell . . . . . . . . . . . . . . . . . . . 53 4.7 Principles of the Silicon Strip Detector . . . . . . . . . . . . . 54 4.8 Setup of the Silicon Strip Detector . . . . . . . . . . . . . . . 55 4.9 Dynamic range extension of the Silicon Strip Detector . . . . 56 4.10 Energy deposition for different particles . . . . . . . . . . . . 57 4.11 Energy response for pions and protons . . . . . . . . . . . . . 57 4.12 Setup of the Scintillating Fiber Tracker . . . . . . . . . . . . 58 4.13 Realization of the Scintillating Fiber Tracker . . . . . . . . . 59 4.14 Detection of light by MAPMT . . . . . . . . . . . . . . . . . 60 4.15 Resistive adaption of MAPMT to ADC . . . . . . . . . . . . 60 4.16 Photo of the CDC-PCB and PFM-PCB . . . . . . . . . . . . 61 4.17 Sketch of the internal circuit of the Gassiplex chips . . . . . . 62 4.18 Sketch of the PFM-PCB . . . . . . . . . . . . . . . . . . . . . 63 4.19 MAPMT response for real signal . . . . . . . . . . . . . . . . 64 4.20 PFM-PCB response for real signal . . . . . . . . . . . . . . . 65 4.21 Sketch of the SFT readout system . . . . . . . . . . . . . . . 66 4.22 Realization of the Photon Detector . . . . . . . . . . . . . . . 67 4.23 setup of the Photon Detector . . . . . . . . . . . . . . . . . . 68 4.24 Components of the Photon Detector . . . . . . . . . . . . . . 68 4.25 Realization of the RD with the Magnet . . . . . . . . . . . . 69 4.26 Recoil Detector installed into the HERMES Detector . . . . . 70 5.1 Illustration of the GSI location . . . . . . . . . . . . . . . . . 72 5.2 Photo of the SFT detector prototype . . . . . . . . . . . . . . 73 5.3 Setup of the readout for the GSI test run . . . . . . . . . . . 74 5.4 SFT readout electronics and sample raw ADC spectrum . . . 75 5.5 Performance of the PID for the GSI test run . . . . . . . . . 75 5.6 Response of a SFT fiber in comparison with simulation . . . . 76 5.7 Performance of pi/p detection . . . . . . . . . . . . . . . . . . 76 5.8 Remaining ambiguity by the readout . . . . . . . . . . . . . . 77 5.9 Setup of the DESY II test beam area for the alignment run . 79 5.10 Setup of the readout for the alignment run . . . . . . . . . . 80 5.11 Internal resolution of the Zeus telesope as reference system . 81 5.12 Performance of a single fiber position reconstruction . . . . . 82 5.13 Illustration of the fiber paths for tweo SFT modules . . . . . 83 5.14 Photo of the installed cosmic ray test run . . . . . . . . . . . 84 5.15 Comparison of old and new backplane PCB . . . . . . . . . . 85 5.16 Hit distribution by cosmic ray of the SFT . . . . . . . . . . . 88 5.17 Cross check of the SFT internal alignment . . . . . . . . . . . 89 5.18 Comparison of reconstructed vertex z-position . . . . . . . . . 93 LIST OF FIGURES 139 5.19 Comparison of reconstructed of transverse polar angle . . . . 93 5.20 Comparison of reconstructed azimuthal angle . . . . . . . . . 94 5.21 Overview of general kinematic variables for DIS events . . . . 96 5.22 Overview of general kinematic variables for DVCS events . . 97 5.23 Distribution of the Mandelstam variable t . . . . . . . . . . . 99 5.24 Comparison of vertex distance to beam axis for 06b2 to 06d0 100 5.25 Small side effect under investigation . . . . . . . . . . . . . . 100 5.26 Amplitude distribution for beam spin asymmetry . . . . . . . 101 5.27 Amplitude distribution for beam charge asymmetry . . . . . . 102 5.28 Performance of particle identification of the SFT . . . . . . . 104 5.29 Correlation between HERMES FS and RD . . . . . . . . . . 105 5.30 p′ difference for FS and RD . . . . . . . . . . . . . . . . . . . φ 106 5.31 Improved background suppression for DVCS events by RD . . 107 6.1 Diagram of GPDs and for DVCS and for BH reactions . . . . 109 6.2 Illustration of the HERMES forward spectrometer . . . . . . 110 6.3 MC Missing invariant mass distribution for different reactions 110 6.4 Realization of the Recoil Detector . . . . . . . . . . . . . . . 111 6.5 Design requirements and expected improvements of the RD . 112 6.6 Preliminary results of the reconstructed azimuthal angle . . . 113 6.7 Amplitude distribution for BSA and BCA . . . . . . . . . . . 113 6.8 Preliminary performance of the particle identification . . . . . 114 6.9 Improved background suppression for DVCS events by RD . . 115 List of Tables 4.1 Summary of the properties of the Recoil Detector . . . . . . . 49 5.1 List of runs of accumulated data samples . . . . . . . . . . . 90 5.2 List of some kinematic cuts for DIS events. . . . . . . . . . . 94 A.1 Node structure for the standalone readout . . . . . . . . . . . 116 A.2 Sequence of nodes for the standalone readout . . . . . . . . . 117 A.3 Signature node of the standalone readout . . . . . . . . . . . 117 A.4 head initiator of the standalone readout . . . . . . . . . . . . 118 A.5 status information of the head initiator . . . . . . . . . . . . . 118 A.6 head terminator of the standalone readout . . . . . . . . . . . 118 A.7 Data initiator of the standalone readout . . . . . . . . . . . . 119 A.8 Status informations of the data initiator . . . . . . . . . . . . 119 A.9 Data for Silicon Strip Detector . . . . . . . . . . . . . . . . . 119 A.10 Data node for the Scintillating Fiber Tracker (ADC part) . . 120 A.11 Data node for the Scintillating Fiber Tracker (TDC part) . . 120 A.12 Status information of the TDC . . . . . . . . . . . . . . . . . 121 A.13 Data node for the Photon Detector . . . . . . . . . . . . . . . 121 A.14 data terminator of the standalone readout . . . . . . . . . . . 121 A.15 Tail initiator of the standalone readout . . . . . . . . . . . . . 122 A.16 Tail terminator of the standalone readout . . . . . . . . . . . 122 A.17 Status informations for the tail terminator . . . . . . . . . . . 122 A.18 Header content of SSD for the experiment readout . . . . . . 123 A.19 Trigger content of SSD for the experiment readout . . . . . . 123 A.20 Trailer content of SSD for the experiment readout . . . . . . 124 A.21 Data content of SSD for the experiment readout . . . . . . . 124 A.22 data overview of SFT for the experiment readout . . . . . . . 124 A.23 Data content of SFT for the experiment readout . . . . . . . 125 A.24 Data Hit content of SFT . . . . . . . . . . . . . . . . . . . . . 125 A.25 Header content of PD for the experiment readout . . . . . . . 126 A.26 Data content of PD for the experiment readout . . . . . . . . 127 A.27 End-of-Block content of PD for the experiment readout . . . 127 140 Acknowledgements At the end of the day it is time to remember..... u First of all I want to express my thanks to Prof. Dr. Michael D¨ren for the rare opportunity to accompany the lifetime of a project of this size in a international collaboration of the first to the latest measurements. I wish everybody this pleasant and constructive atmosphere in which I was allowed to contribute by my work. Moreover I want to thank the european graduate school for the scientific envelop of my stay in Giessen and all the lecture weeks for exchange with other colleagues. I want to thank Shaojun for the great help and to share the office with me. As well many thanks have to delivered to Bjoern, Hasko, Matthias, Matthias and Lukas of the dark old days in the group for the numerous discussions, feedbacks and supports. Without their help and great effort the preparation of the detector would not be possible. Furthermore I want to thank Markus, Roberto and Weilin for the company of the exiting time and for the fine work during all these heavy commissioning phases. Without many help and advices by my colleagues Avetik and Peter the finalization of the writeup of the dissertation and preparation of the talk for the disputation would take much longer. A good administration can be seen that one does not note them. There- fore thanks a lot to Marianne and Anita to keep all those work apart. As well I want to thank Juergen and Werner for the fine organizational support and for all the tasty breakfast. Such a project can only be done with a lot of help by the mechanical workshop of Rainer and the electronic workshop of Cristoph. Many thanks I want to give to their colleagues as well. Without the fast and fine help of the HADES group especially of Michael Boehmer and Roman Gernhaeuser the readout of the detector would not be that stable and reliable. Therefore I want to thank them all. During the preparation of the detector many important and successful test runs were performed before the detector was installed. Therefore I want to thank R.Simon and D.Schardt for the evaluations at GSI. Many thanks I have to apply to Norbert and his colleagues at DESY for the alignment run in the test beam area. For the run of the cosmic ray test and for the installation of the detector a lot of work and help was done by Carsten, Helmut, Ingrid, Kurt, Volker and Yorck. I want to give many thanks to them and their colleagues behind for the fine and fast support. The HERMES secretary by Sabine, Soerne and Evelyn was always a supportive contact. Thanks a lot for all the help and fun. A lot of thanks to all these ancient people I do not know for their permanent push to bring this fine HERMES experiment up. My contribution to the HERMES collab- oration would not be possible without the patience and support by Vitaly for the implementation of the additional readout into the HERMES DAQ 141 and by Iouri and Valdimir for the implementation of the recoil trigger part. Furthermore I want to thank Prof. Toshi-Aki Shibata to support me for the FGIP to stay in Japan. It was a very educational time to extend the horizon and to share time with his students. I have many in the debt to an outnumbered amount of people at HERMES who had incredible amount of patience and support to me in such a great international collaborations. Sorry it is not intentional but I am afraid I will not remember all of them anymore. Nevertheless many, many thanks to those like Alexander, Andy, Avetik, Beni, Elke, Jim, Larry, Ralf, Yorck and to the dedicated recoil group members too. I want to thank all the people for the time during offsite activities. Spe- cial thanks I want to give Andrzej, Bohdan and the others for all the relaxed time of cooking and discussions. During the run time of the experiment I had a very fine lodging time during my stay at Carsten's home. I want to express my thanks to him, Gisa, their family and their friends. It was always a very entertaining and relaxing occasion to spend my time at their activities. A lot of thanks belongs to my family as well. My parents, especially for my mother, I fell great respect to make it possible to me to do all these studies and works. I have a rough idea about the difficulties to parent alone two kids in a foreign country out of nothing, chapeau. As well many thanks to my sister and her family for the permanent motivation and help. Finally I want to thank all those out there to keep this space ship earth running......... P.S. At the very end I want to thank Prof. Dr. Claus-Dieter Kohl to be the second referee and another examiner and furthermore to PD. Dr. Stefan Leupold to be the additional examiner of my disputation. 142