Photon-Jet Correlations at RHIC
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Photon-Jet Correlations at RHICSaskia MioduszewskiTexas A&M University 18 July, 2007
OutlineWhat can be learned from g-jet measurements?
How to measure?
Current Status of Measurement at RHIC
Why g-jet?qqgqqgComptonAnnihilationGolden Probe of energy loss - calibrated probe of jet energy - X.-N. Wang et al.Gluon jet vs. quark jet energy loss compare g-jet to p0-jet (in g-jet, the jet will mostly be from quarks) - need to understand the surface bias to interpret data !
What can be learned?Ultimate goal is to understand energy loss mechanism
Complementary to single particle spectra and di-jets
No surface bias for trigger particle, different surface bias for g-jet coincidence
Single-Particle Spectra What have we learned?Hadrons are suppressed by factor ~5, Photons are not
Single-Particle Spectra What have we learned?Hadrons are suppressed by factor ~5, Photons are not PHENIX, Phys. Rev. Lett. 96, 202301 (2006)
Di-Jets through Hadron-Hadron CorrelationsDisappearance of away-side jet in central Au+Au collisions4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig0-5%STAR, PRL 90 (2003) 082302
Reappearance of away-side jetWith increasing trigger pT, away-side jet correlation reappearsSTAR, Phys. Rev. Lett. 97 (2006) 162301Higher pT JetNear SidePunch-through Jet?
Reappearance of away-side jetWith increasing trigger pT, away-side jet correlation reappearsSTAR, Phys. Rev. Lett. 97 (2006) 162301Tangential emission?
Surface Bias of Di-Jets?8 < pT,trig< 15 , 4< pT,assoc< 6 GeV/c8 < pT,trig< 15 GeV/cSTAR, Phys. Rev. Lett. 97 (2006) 162301
What can be learned?
Di-jets suffer surface bias of trigger jet, punch-through of away-side (or only tangential emission) g-Jet trigger photon has no bias, but existence of away-side jet creates a (different) bias Measurement of % transmission as a function of true jet energy
Challenging MeasurementTrigger photon measured with electro-magnetic calorimeterLarge background from hadronic decays (p0, h) Background sources of photons (hadronic decays) suppressed in central Au+Au collisionsPHENIX, Phys. Rev. Lett. 94, 232301 (2005)
g-jet Analysis in Au+Au collisionsThomas Dietel, Quark Matter 2005
Challenging MeasurementTrigger photon measured with electro-magnetic calorimeterLarge background from hadronic decays (p0, h)
Background signal is di-jet (p0-jet)Subtract from inclusive signalNeed large-statistics data set for g-jet measurement
Statistical measurement of g-jet yieldsMeasure inclusive g-jet correlation functionMeasure p0-jet (h-jet) correlation functionConstruct resulting gdecay-jet correlation taking into account decay kinematicsSubtract gdecay-jet from ginclusive-jet Result is gdirect-jet
PHENIX g-jet in p+pPHENIX PreliminaryPHENIX PreliminaryJ.Jin, QM 2006Comparison to PYTHIA verification of method
Direct g-h Yields
Signature small near-side correlation signal apparent Yield sensitive to h contribution at the near-sideNew h corrected results consistent with QM result within systematic errorStill room for some fragmentation contributionRun 5 p+p @ 200 GeV5-7 X 1-2 GeV/c7-9 X 1-2 GeV/c5-7 X 2-5 GeV/c7-9 X 2-5 GeV/c9-12 X 2-5 GeV/c9-12 X 1-2 GeV/cM. Nguyen, APS 07
PHENIX g-jet in Au+Au collisionsJ.Jin, QM 2006Black: Au+Au Red: p+p
Cu+Cu gdirect-h+/- vs. p0-h+/- jet yieldsper-trigger jet pair yields:systematic from Rgsystematic from subtraction methodA. Adare, WWND 2007
Distinguishing direct photons from p0Opening angle of p0 as a function of pT Segmentation of calorimeter towers in STAR (Dj,Dh) = (0.05,0.05) 2 decay photons hit same tower starting pT ~ 5 or 6 GeV/c Segmentation in PHENIX ~ 0.01 radians
Distinguishing direct photons from p0Reject clusters that look like 2 close photons (from p0 decay) from shower shape
Retain sample of correlations that are gdirect-jet rich
Estimate remaining contamination from p0-jet
Measure and subtract p0-jet away-side yield
Distinguishing single photons from 2 decay photons in STARShower-max detector(Dh,DF)~(0.007,0.007) Hamed, Collective Dynamics 2007
STAR g-jet in p+pSubhasis Chattopadhyay, QM 2006-- Improved p0/g separation using shower profile in SMD
STAR g-jet in p+pComparison to HIJING p+p verification of methodSubhasis Chattopadhyay, QM 2006Ettrigger from 8.5GeV/c to 10.5 GeV
Next StepsDo similar analysis for Au+Au collisions as was done in p+p and d+Au Challenge within large-multiplicity backgroundRun-7 results coming soonResults must be interpreted together with di-jet results and/or within model to take surface bias into account
SummaryDi-Jets surface bias results indicate jets are either transmitted with negligible energy loss or absorbed completelyPhoton-Jet Measurement will complement the di-jet for more complete probeA significant (statistically and systematically) g-jet Measurement in Au+Au is neededNeed a great deal of statistics!Stay tuned for the results from Run-7. Increase in statistics over Run-4 Au+Au > factor of 10