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The STAR Experiment Quark Matter 2009 Knoxville, TN Texas A&M University A. M. Hamed for the STAR collaboration 1 Outline Motivation Analysis Technique Status QM08 and HP08 New Results d+Au enhanced kinematic reach Discussion and summary 2 Energy loss via gamma-jet coincidence oDirect photon energy balances the outgoing parton, modulo correction from constrain kinematics. initial state radiation. Motivation: oAccurate measurements of energy dependence of energy loss (-jet in AuAu and pp) oPossible measurements of path length dependence of energy loss (-jet vs 0-jet) h h 0 LO production mechanisms: Away side yield per direct photon vs. per pi0 O(αs) 1. Geometrical bias Photon is electromagnetically interacting particle, then 0 -triggers are surface biased compared to direct photon. On average the away side of pi0 trigger suffer from Compton longer path length unless it is tangential emission Ahmed M. Hamed QM2009 Knoxville, Tennessee Annihilation 3 Energy loss via gamma-jet coincidence 2. Away side parton type Away side of direct photon trigger in the dominant channel is always quark vs. quark/gluon mix in pi0 trigger a. Gluon jet fragment softer than quark jet at the same energy b. Gluon is bi-colored (CA=3, CF=4/3) “color factor” 3. Initial parton energy 0 -triggers are resulted from higher parton energy than -triggers. Associated away side yield per pi0 trigger is higher than this from gamma trigger at the same energy More away side yield is associated with pi0 trigger than this from direct gamma trigger Higher order processes: kT Photon Bremsstrahlung Ahmed M. Hamed O(αs2) Example of Bremsstrahlung diagrams QM2009 Knoxville, Tennessee 4 Analysis: Analysis technique Build correlation function for neutral “triggers” with “associated” charged particles 0 rich Use transverse shower profile to distinguish 2-photon “0” from single-photon showers “rich” The fact that direct photon has zero near side yield : away side yield per trigger associated with direct photon : away side yield per trigger associated with pi0 Ahmed M. Hamed QM2009 Knoxville, Tennessee 5 Away side results Y+h Y0+h Shown results Path length dependence of energy loss oY+h oY0+h pp, run’06 11 pb-1 AuAu, run’07 535 ub-1 dAu, run’08 Energy dependence of energy loss 6 Status QM’08 and HP’08 Direct gamma – charged hadron coincidence at high pT The associated yield with direct gamma trigger agrees with theoretical model predictions. h 0 h h h 0 h Similar suppression in IAA , IAA, IAA, RAA and RAA Ahmed M. Hamed QM2009 Knoxville, Tennessee 7 New Results : neutral pions yield Y0+h D(zT) = 0 N The first measurements at RHIC energy for FF cover this wide range of kinematics in pp and dAu of 0-hadron coincidence Good agreement between data from different runs and different systems at STAR detector Yields show significant medium effect Ahmed M. Hamed QM2009 Knoxville, Tennessee 8 New Results : neutral pions IAA The per trigger yield suppression is final state effect. Quantifying the suppression -jet coincidence measurements Ahmed M. Hamed QM2009 Knoxville, Tennessee 9 New Results : Direct photon yield Y+h D(zT) = N The first measurements at RHIC energy for FF cover this wide range of kinematics in pp and dAu of -hadron coincidence Yields show significant medium effect FF of -jet coincidence is in well described by theory Ahmed M. Hamed QM2009 Knoxville, Tennessee 10 New Results : Direct photon in dAu Good agreement between pp and dAu data for and 0 results The modification in the FF of -jet coincidence measurements is final state effect Associated yield for direct photons is significantly suppressed compared to that of 0 due to the difference in the parton initial energy. Ahmed M. Hamed QM2009 Knoxville, Tennessee 11 New Results : IAA for and 0 theory vs. data No agreement bewteen different theoretical calculations and no strong raise on IAA at ZT < 0.6 as expected by one of the theoretical calculation. No differences between IAA and 0 results which indicate that the surface bias is driven by the near side trigger only. See STAR talk on Full jet reconstruction Path length energy loss dependence is better to be explored by -jet in- and out- of the reaction zone. Ahmed M. Hamed QM2009 Knoxville, Tennessee 12 New Results : Direct photon IAA Theory describe the data within the measured uncertainties. Other centralities show similar IAA(pt) within current uncertainties which contradict the picture of volume and surface emission. Ahmed M. Hamed QM2009 Knoxville, Tennessee 13 Summary and Discussion An accurate measurement for the energy dependence of energy loss of the propagating parton through the medium is shown via -jet coincidence measurements. Continuous effort is currently undertaking to improve the uncertainties levels. The similarity between -jet and 0-jet indicate the free geometrical bias of the recoil parton. -jet coincidence measurements are needed to explore the path length dependence of the energy loss. 14