Abstract
to improve and verify the trigger efficiency, the reconstruction algorithms, the detector calibration and stability, and the particle identification efficiency. A review of these aspects will be given. Fig. 1 ). The use of PbW04 crystals has allowed View of the CMS ECAL structure: Barrel (one supermodule in yellow), Endcap (in green), Preshower (in red).
the design of a compact calorimeter inside the solenoid that is fast, has fine granularity, and is radiation resistant. The Barrel section (EB) has an inner radius of 129 cm, and is structured as 36 identical "supermodules", each covering half the barrel length and corresponding to a pseudorapidity interval of 0 < 1'1]1 < For electromagnetic showers of energies above 100 Ge V the energy resolution is dominated by the constant term.
As a consequence, in the CMS environment the detector's performance will depend mainly on the quality of its inter calibration and monitoring [7] .

with pp collisions at VB = 7 TeV will be reviewed, and to some extent for the 2011 running as well. Pure samples of electrons and photons from decays of known resonances have been exploited to improve and verify the trigger efficiency, the reconstruction algorithms, the detector calibration and stability, and the particle identification efficiency. A review of these aspects will be given.
I. DESCRIPTION OF THE DETECTOR
T HE Compact Muon Solenoid (CMS) [1] electromagnetic calorimeter (ECAL) [2] at the Large Hadron Collider (LHC) [3] is an hermetic, homogeneous calorimeter compris ing 61200 lead tungstate (PbW04) crystals mounted in the central Barrel part, closed by 7324 crystals in each of the two Endcaps (see Fig. 1 ). The use of PbW04 crystals has allowed View of the CMS ECAL structure: Barrel (one supermodule in yellow), Endcap (in green), Preshower (in red).
the design of a compact calorimeter inside the solenoid that is fast, has fine granularity, and is radiation resistant. The Barrel section (EB) has an inner radius of 129 cm, and is structured as 36 identical "supermodules", each covering half the barrel length and corresponding to a pseudorapidity interval of 0 < 1 ' [4] .
The ECAL detector has a crucial role in the physics program of CMS. Its design has been driven by the requirement of an excellent energy resolution [5] . This is needed in particular for the search of the Higgs boson in the H -+ "(Y decay channel, one of the primary goals of the LHC physics program, where a good di-photon invariant mass energy resolution is essential to discriminate a signal with a small cross section over a large irreducible Standard Model background.
The ECAL energy resolution measured in electron test beams is parametrized as
for electrons incident on the center of crystals [6] . For electromagnetic showers of energies above 100 Ge V the energy resolution is dominated by the constant term.
As a consequence, in the CMS environment the detector's performance will depend mainly on the quality of its inter calibration and monitoring [7] .
II. STATUS AND STABILITY
During the first months of collisions at LHC, in 2010 and 2011, the percentage of fully working channels in the ECAL Barrel and Endcap detectors is about 99.2% and 98.7%, respectively. For the Preshower, the same percentage is around 95.1 %. The stability of the calorimeter as a whole is crucial to reach the design goal for the constant term in the energy resolution. In particular, the monitoring of the temperature of the ECAL detectors is very important, since any change affects both the crystal transparency (-2.2%;oC) and the APD gain (-2.4%j DC). The calorimeter has thus been instrumented with nearly 7000 thermistors to provide accurate monitoring. Fig. 2 shows the RMS deviation of the temperature measurements for each sensor: the results are well within the stringent requirements (O .osoe for the Barrel and 0.1 DC for the Endcap) imposed to achieve a constant term in the energy resolution of 0.5% [7] . impinging on the centre of a Barrel crystal is contained within that crystal), to achieve the best estimate of the energy of the incoming particle, algorithms for clustering the energy deposits have been developed. The energy of electrons and photons can thus be expressed as:
xtal where the sum is performed over the crystals in a cluster, Axtal is the measured APD'sNPT's raw ADC amplitude, Cxtal is the crystal inter-calibration coefficient, Lxtal (t) is the time dependent correction for the radiation-induced transparency loss, G is the ADC-to-GeV energy scale, and Fe,! is an additional correction factor, which depends on the type of the particle and on its energy, and takes into account geometry and material effects like shower leakage and bremsstrahlung losses.
The ECAL detector has been calibrated prior to the LHC startup with an overall precision of 0.5% -2% in the Barrel, and 5% in the Endcap, using a combination of laboratory measurements of the light yield and photodetector gain (with a precision of 4.5% -6%), electron test beam measurements (for 25% of the channels in the Barrel, with a precision of 0.5%), and muons from cosmic rays (with a precision of 1.4% -3.5%) and beam dumps (with a precision of about 6%). Before the installation in CMS, the global energy scale has been defined using test beam data, and then tuned by reconstructing di-photon and di-electron invariant mass peaks of known particles with unconverted photons and electrons with very low bremmstrahlung [7] .
A. Inter-calibration
The start-up calibration precision is improved using proton proton collision data, combining different methods [9] :
• the azimuthal symmetry method, exploiting the energy flow invariance around the beam axis in minimum bias events, to inter-calibrate crystals at the same pseudora pidity;
• 7r0 and 'TJ resonances inter-calibration, using the invariant mass peaks of di-photon events from 7r0 ('TJ) � "
candidates (also useful to monitor the energy scale);
• isolated, high energy electrons from W �ev and Z�ee decays, comparing the energy measured in ECAL with the track momentum measured in the CMS silicon Tracker;
• radiative Z decays, using Z� 1111" assuming the correct muon energy scale, to check the photon energy scale. Calibration precision from the azimuthal symmetry method, from minimum-bias events, for the ECAL Barrel detector.
obtained combining all the different calibration strategies, resulting in a precision of less than 0.5% at low values of I 'TJ I, and increasing due to the material in front of the detector for 11]1 > 1 in the ECAL Barrel, and less than 2% for 1.65 < 11]1 < 2.6 in the Endcap, corresponding to the region covered by the Preshower detector. 
B. Crystal transparency monitoring
The design energy resolution of the ECAL detector depends on the accurate measurement and correction of the time dependent changes in the crystal light yield caused by light transmission losses due to radiation damage, Lxtal(t) [6] . The Tracker, for electrons from W -+ell decays, before and after the applying the transparency loss corrections. The ratio is normalized to unity at the beginning of the period considered.
The uncorrected ratio shows a drop of 1.5% in the Barrel detector (and correspondingly 6% in the Endcap detector) during this period. After correction, the E / p histories are flat to within 0.2% and 1%, respectively. E/p history plot, with E = ECAL electron energy, p = Tracker electron momentum, for W-+ev decays measured in 2010 with the ECAL Barrel detector.
IV. ENERGY SCALE
The energy scale, G, initially set using electron test beam measurements, has been tuned using Z-+ee events from LHC collisions for both the ECAL Barrel and Endcap detectors [4] . Fig. 9 . Estimation of the photon energy scale from Z-+ /-L/-L"I events, for photons in the ECAL Barrel (left) and in the Endcap (right) detectors.
V. ENERGY RESOLUTION
The energy resolution performance has been measured with an electron test beam in the energy range between lO Ge V and 180 Ge V [10] . After the installation, the resolution has been studied using di-photon pairs from 7[0 decays and Z-+ee events produced in proton-proton collisions [6] . fitted to a convolution of a Breit-Wigner (BW) and a Crystal
Ball (CB) function using an unbinned likelihood fit, to extract the Z peak shape parameters from the values of !af76:e4bb:d472:9fe5:10c0:37a3:766a:5b99l. mcB, the difference (in GeV ) between the CB mean and the Z mass [11] , and aCB, the width of the CB function. The BW parameters are fixed to their PDG values [11] . There is a good agreement between the CB width for data and Monte Carlo distributions,
showing the good detector modeling in the simulation, with the CB width being around 1 Ge V for non-showering electrons.
VI. SUMMARY
The CMS ECAL detector status and performances with the first proton-proton collisions at the Large Hadron Collider have been presented. The results match extremely well the expectations, thanks to the impressive preparatory work, and are close to the design goals.
ACKNOWLEDGMENT
The author would like to thank the organizers of this
Conference for their invitation to come to the beautiful city of
Valencia, and present the CMS ECAL detector status.

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