Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR
Zilong Chang







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#1#2#3#4#1 #2, #3 (#4)


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Nucl. Phys. B
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Phys. Rev. Lett.
Phys. Rev. D
Z. Phys. C
J. High Energy Phys.





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    CP-1.80em/
        
        
        
        
        
        
        Department of Physics, Brookhaven National Laboratory,
        
        Upton, NY 11973Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR
            Zilong Chang for the STAR Collaboration
            
        ==============================================================================================
        
        
        Jets produced in the pseudo-rapidity range, -1.0 < η < 1.0, from pp collisions at RHIC kinematics are dominated by quark-gluon and gluon-gluon scattering processes. Therefore the longitudinal double spin asymmetry A_LL for jets is an effective channel to explore the longitudinal gluon polarization in the proton. At STAR, jets are reconstructed in full azimuth, from the charged-particle tracks seen by the Time Projection Chamber and electro-magnetic energy deposited in the Barrel and Endcap electro-magnetic calorimeters at both √(s) = 200 and 510 GeV. Early STAR inclusive jet A_LL results at √(s) = 200 GeV provided the first evidence of the non-zero gluon polarization at x > 0.05. At  √(s) = 510 GeV, the inclusive jet A_LL is sensitive to the gluon polarization as low as x ∼ 0.015. In this talk, we will discuss recent STAR inclusive jet and dijet A_LL results at √(s) = 510 GeV and highlight the new techniques designed for this analysis, for example the underlying event correction to the jet transverse energy and its effect on the jet A_LL. Dijet A_LL results are shown for four topologies in regions of pseudo-rapidity, effectively scanning the x-dependence of the gluon polarization.     
        
        
        § INTRODUCTION
        
        
        The spin of the proton has been known as 1/2, however the early deep inelastic scattering, DIS, experiments showed that quarks inside the proton contribute about 30% of its total spin <cit.>. Where the rest of the proton spin comes from has been puzzling for the past three decades. Neither inclusive or semi-inclusive measurements from DIS experiments offer a good description of the gluon contribution <cit.>.
        
        Different from DIS experiments, hadronic collisions such as proton-proton, pp, collisions provide an effective environment to study the gluon polarization inside the proton. In longitudinally polarized pp collisions, the double spin asymmetry, A_LL, for jets, defined as the fractional difference of the jet cross section when two beams have the same and the opposite helicities, is proportional to the partonic double spin asymmetry, â_LL, convoluted with the polarized parton distribution functions, PDFs, of the scattering partons. At center of mass energy √(s) = 200 and 510 GeV, the jet production is contributed by hard quantum chromodynamics, QCD, scattering processes. As seen from Figure <ref>, qg and gg processes dominate the jet productions at jet x_T = 2p_T/√(s) < ∼ 0.3, where the cross section is appreciable <cit.>. Both processes have sizable partonic â_LL, therefore jet A_LL is sensitive to gluon polarization.
        
        
        
        The Relativistic Heavy Ion Collider, RHIC, is the one and only hadronic collider capable of colliding polarized proton beams up to √(s) = 510 GeV <cit.>. A hydrogen gas-jet polarimeter <cit.> and a Coulomb nuclear interference proton-Carbon polarimeter <cit.> located at 12 o'clock in the ring provide the beam polarizations. The Solenoidal Tracker at RHIC, STAR, sitting at 6 o'clock is a symmetric detector in full azimuth, 0 < ϕ < 2π<cit.>, including, a time projection chamber, TPC, at |η| < 1.3 <cit.>, barrel and endcap electromagnetic calorimeters, BEMC and EEMC, |η| < 1.0 and 1.0 < η < 2.0 respectively, <cit.> and a forward meson spectrometer, FMS, at 2.6 < η < 3.9. In addition, the vertex position detector, VPD <cit.>, and the zero degree calorimeter, ZDC <cit.>, are used to provide relative luminosity and local polarimetry monitoring.
        
        STAR has published a series of results on both inclusive and dijet A_LL over the past two decades at √(s) = 200 GeV <cit.>. The DSSV group who included the two recent STAR dijet A_LL results, one with both jets in |η| < 0.8<cit.> and the other with at least one jet in 0.8 < η < 1.8<cit.>, shows the gluon polarization, ∫_0.01^1Δ g(x) = 0.296 ± 0.108 at Q^2 = 10 GeV^2<cit.>.
        
        
        
        § JET RECONSTRUCTION FROM 510 GEV DATA
        
        
        In the years 2012 and 2013, STAR has taken data from 510 GeV longitudinally polarized pp collisions. The beam polarizations for both years are between 50% to 53%<cit.>. The integrated luminosity for the 2013 run is about 3.7 times larger than that for the 2012 run, however the jet trigger in the 2013 run is optimized for the dijet measurements.
        
        Jets are reconstructed from charged particles measured by the TPC and electro-magnetic energy collected by BEMC and EEMC towers, a 0.05 × 0.05 space in η-ϕ. Due to large pile-up event and soft background environment at √(s) = 510 GeV, anti-k_T algorithm with jet parameter R = 0.5 was used in this analysis <cit.>.
        
        A technique called the off-axis cone method was introduced to correct the jet transverse energy due to underlying event activities <cit.>. The method collects the particles inside two off-axis cones centered at the jet η and ±π/2 off the jet ϕ and finds the average transverse energy density of the two cones. The correction, dp_T, is taken as the product of the jet area and the found cone energy density. It allows one to study the η dependence of the underlying event activities. Furthermore with this method, the underlying event contribution to the jet A_LL can be estimated.
        
        In order to study the systematic uncertainty, an embedding sample was generated by embedding simulated QCD events into zero-bias events, randomly taken during the real collisions. GEANT was used to simulate the detector response <cit.>. Intermediate particles and partons in the simulation are saved as well to quantify the correction from the measured detector jet quantity to the parton jet quantity, which facilitates comparisons with theoretical calculations.
        
        The Perugia 2012 tune from PYTHIA6 <cit.> overestimates the charged pion, π^±, yields at low transverse momentum, p_T, comparing to previously published STAR measurements <cit.>. By reducing the PARP(90) parameter from 0.24 by default to 0.213, which controls the √(s) dependence of the cut-off p_T,0, where it regulates the low p_T divergence of the partonic cross section, a better agreement is achieved <cit.>. Figure <ref> shows an excellent agreement between data and embedding for the jet transverse momentum, p_T spectrum satisfying three jet-patch triggers separately <cit.>.
        
        
        
        § INCLUSIVE JET AND DIJET A_LL
        
        Experimentally, the jet A_LL is calculated from the difference in the number of jets measured when beams have the same and the opposite helicities, N^++ and N^+- respectively, by taking into account the relative luminosity, R and beam polarizations, P_1 and P_2, as in Eq. <ref>:
        
        
        A_LL = 1/P_1P_2N^++ - RN^+-/N^++ - RN^+-.
        
        
        Similarly the longitudinal double spin asymmetry for the underlying event correction dp_T, A_LL^dp_T, can be defined as the fractional difference of the averaged dp_T for jets found when beams have the same and the opposite helicities as in Eq. <ref>. Figure <ref> shows the A_LL^dp_T is consistent with zero <cit.>. Its numerical value is used to calculate the potential impact the underlying events have on the longitudinal double spin dependent jet cross section, σ^++ and σ^+-. It turns out the the underlying event activity contribution to the jet A_LL is at the level of 10^-4, assigned as a systematic uncertainty.
        
        
        A_LL^dp_T = 1/P_1P_2<dp_T>^++ - <dp_T>^+-/<dp_T>^++ + <dp_T>^+-
        
        The right panel of Figure <ref> shows the inclusive jet A_LL compared to the STAR 2009 results at √(s) = 200 GeV <cit.> as a function of jet x_T. The two results at different √(s) agree well with each other, while the new techniques developed in this analysis lead to much reduced systematic uncertainty than the previous measurements. The 510 GeV results are also consistent with recent polarized PDF predictions <cit.>. As seen in the left panel of Figure <ref> the sampled x_g can reach as low as 0.015 <cit.>.
        
        
        
        The initial parton kinematics, x_1 and x_2, can be calculated from the kinematics of the measured dijet events. In this analysis, the dijet A_LL are measured in four distinct topologies based on the η range of the two jets, as listed in Table <ref>. Each topology features its own sampled x_1 and x_2 distributions, therefore the sampled parton x distributions are much narrower than that sampled by inclusive jets. The A_LL also varies with topologies because the partonic â_LL depends on the scattering angle cosθ^*. The dijet A_LL and its sampled x_1 and x_2 distributions can be seen in Figure <ref><cit.>.
        
        
        
        The preliminary results of STAR 2013 inclusive and dijet A_LL at √(s) = 510 GeV <cit.> are consistent with the 2012 results. The systematic uncertainties are under further study for future publication.
        
        
        
        § OTHER MEASUREMENTS AND STAR FORWARD UPGRADE
        π^0s are recontructed from FMS in two η ranges, 2.65 < η < 3.15 and 3.15 < η < 3.90. The inclusive π^0A_LL as a function of transverse momentum is found to be very small, less than 5 × 10^-3. The A_LL will help to constrain the gluon polarization at x_g ∼ 10^-3<cit.>.
        
        STAR is preparing for an upgrade in the forward region, 2.8 < η < 3.7, which requires the installation of a forward calorimeter and a forward tracking system in time for the 2022 RHIC run <cit.>. The calorimeter, placed at the current FMS location, includes both a hadron calorimeter and an electro-magnetic calorimeter. Three silicon ministrip detectors combined with four small strip thin gap chambers, sTGC, provide the tracking system. The dijet A_LL will be one of the highlighted measurements for the upgrade with one or both of the two jets in the forward region. The predicted precision will be around 5 to 6 times better than the current uncertainty band at invariant dijet mass less than 20 GeV/c^2 when both jets are detected in the forward region.
        
        
        § CONCLUSION
        
        STAR inclusive jet and dijet A_LL measurements are unique and can be used to unravel the polarized gluon PDF in the proton. The inclusive jet measurements probe the magnitude of the gluon polarization, while the dijet measurements constrain the shape of Δ g(x). The previous results at √(s) = 200 GeV provided the first evidence of positive gluon polarization at x > 0.05, furthermore at √(s) = 510 GeV, the jet A_LL will enable access to Δ g(x) at x around 0.015. We expect future results with more precision at both √(s). In the near future, the forward upgrade will explore small x gluon polarization as low as 10^-3.
        
        
        § REFERENCES
        99emc1988 J. Ashman et al. (EMC Collaboration), 2063641988.
        
        lss10 E. Leader, A. V. Sidorov, and D. B. Stamenov 8211408122010.
        
        nlo2012 A. Mukherjee and W. Vogelsang 860940092012.
        
        rhic M. Harrison, T. Ludlam, and S. Ozaki 4992352003.
        rhicdesign H. Hahn  et al.4992452003.
        rhicpol I. Alekseev  et al.al.4993922003.
        hjet H. Okada  et al. arXiv:hep-ex/0601001.
        pCpol O. Jinnouchi  et al. arXiv:nucl-ex/0412053.
        star K. Achermann  et al.4996242003.
        tpc M. Anderson  et al.4996592003.
        bemc M. Beddo  et al.4997252003.
        eemc C. Allgower  et al.4997402003.
        vpd  W. J. Llope  et al.759232014.
        zdc  C. Adler, A. Denisov, E. Garcia, M. Murray, H. Strobele,
          and S. White  et al.4704882001.
        run9aLL L. Adamczyk  et al. (STAR Collaboration), 1150920022015.
        run9bdj L. Adamczyk  et al. (STAR Collaboration), 95071103(R)2017.
        run9edj J. Adam  et al. (STAR Collaboration), 980320112018.
        dssv2019 D. de Florian, G. A. Lucero, R. Sassot, M. Stratmann, and
          W. Vogelsang, arXiv:1902.10548 [hep-ph].
          polnote W. B. Schmidke  et al. Report No. BNL-209057-2018-TECH, 2018.
        antikt M. Cacciari, G. P. Salam, and G. Soyez, 040632008.
        cones Abelev B,  et al (ALICE Collaboration) 911120122015.
        geant R. Brun, F. Bruyant, F. Carminati, S. Giani, M. Maire, 1288
        A. McPherson, G. Patrick, and L. Urban, Reports No. CERN-W5013, 1994.
        pythia T. Sjostrand, S. Mrenna, and P. Skands, 050262006.
        perugia P. Skands, 820740182010 and arXiv:
          1005.3457v5.
        pipm2006 J. Adams  et al. (STAR Collaboration), 6371612006.
        pipm2012 G. Agakishiev  et al. (STAR Collaboration), 1080723022012.
        adkins  J. K. Adkins, Ph.D. thesis, University of Kentucky, 2017.
        chang Z. Chang, Ph.D. thesis, Texas A&M University, 2016.
        run12aLL J. Adams  et al. (STAR Collaboration), 1000520052019.
        dssv2014 D. de Florian, R. Sassot, M. Stratmann, and W. Vogelsang, 1130120012014.
        nnpdf1.1 E. R. Nocera, R. D. Ball, S. Forte, G. Ridolfi, and J. Rojo
        (NNPDF Collaboration), 8872762014.
        run13aLL A. Quintero for the STAR Collaboration, arXiv:1809.00923 [nucl-ex].
        fmspi0 J. Adam  et al. (STAR Collaboration), 980320132018.
        fwdupg https://drupal.star.bnl.gov/STAR/starnotes/public/sn0648.
