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Photon-Jet Correlations at RHIC

This presentation explores the significance of gluon-jet measurements in understanding energy loss mechanisms at the Relativistic Heavy Ion Collider (RHIC). It discusses the methodology for gauging these measurements and outlines the current state of experimental data. Key findings include the differential suppression of hadrons compared to photons, the analysis of di-jet correlations, and the implications of surface bias in jet observations. The ultimate aim is to deepen our understanding of energy loss in high-energy nuclear collisions, offering critical insights into quark-gluon plasma behavior.

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Photon-Jet Correlations at RHIC

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  1. Photon-Jet Correlations at RHIC Saskia Mioduszewski Texas A&M University 18 July, 2007

  2. Outline • What can be learned from g-jet measurements? • How to measure? • Current Status of Measurement at RHIC

  3. Dominant g q γ q g γ q g q Bremsstrahlung Why g-jet? q γ γ q “Golden 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 ! g q g q Compton Annihilation

  4. 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

  5. Single-Particle Spectra – What have we learned? Hadrons are suppressed by factor ~5, Photons are not

  6. PHENIX p0 Spectrum PHENIX preliminary Fractional “effective” energy loss Sloss ~ 20% in most central Au+Au collisions Single-Particle Spectra – What have we learned? Hadrons are suppressed by factor ~5, Photons are not PHENIX, Phys. Rev. Lett. 96, 202301 (2006)

  7. Escaping Jet “Near Side” Lost Jet “Far Side” pedestal and flow subtracted Di-Jets through Hadron-Hadron Correlations “Disappearance of away-side jet” in central Au+Au collisions 4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig 0-5% STAR, PRL 90 (2003) 082302

  8. “Reappearance of away-side jet” With increasing trigger pT, away-side jet correlation reappears STAR, Phys. Rev. Lett. 97 (2006) 162301 Higher pT Jet “Near Side” “Punch-through” Jet?

  9. “Reappearance of away-side jet” With increasing trigger pT, away-side jet correlation reappears STAR, Phys. Rev. Lett. 97 (2006) 162301 Tangential emission?

  10. Renk and Eskola, hep-ph/0610059 BDMPS-style energy loss + evolving bulk system Surface Bias of Di-Jets? STAR, Phys. Rev. Lett. 97 (2006) 162301 8 < pT,trig< 15 GeV/c 8 < pT,trig< 15 , 4< pT,assoc< 6 GeV/c

  11. q g 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

  12. Challenging Measurement • Trigger photon measured with electro-magnetic calorimeter • Large background from hadronic decays (p0, h) PHENIX, Phys. Rev. Lett. 94, 232301 (2005) • Background sources of photons (hadronic decays) suppressed in central Au+Au collisions

  13. g-jet Analysis in Au+Au collisions Thomas Dietel, Quark Matter 2005

  14. Challenging Measurement • Trigger photon measured with electro-magnetic calorimeter • Large background from hadronic decays (p0, h) • Background signal is di-jet (p0-jet) • Subtract from inclusive signal • Need large-statistics data set for g-jet measurement

  15. Statistical measurement of g-jet yields • Measure inclusive g-jet correlation function • Measure p0-jet (h-jet) correlation function • Construct resulting gdecay-jet correlation taking into account decay kinematics • Subtract gdecay-jet from ginclusive-jet  Result is gdirect-jet

  16. PHENIX g-jet in p+p J.Jin, QM 2006 PHENIX Preliminary PHENIX Preliminary Comparison to PYTHIA – verification of method

  17. Direct g-h Yields Run 5 p+p @ 200 GeV M. Nguyen, APS 07 • Signature small near-side correlation signal apparent • Yield sensitive to h contribution at the near-side • New h corrected results consistent with QM result within systematic error • Still room for some fragmentation contribution 9-12 X 1-2 GeV/c 5-7 X 1-2 GeV/c 7-9 X 1-2 GeV/c 9-12 X 2-5 GeV/c 5-7 X 2-5 GeV/c 7-9 X 2-5 GeV/c

  18. 1/NtrigdN/dDf Df(rad) PHENIX g-jet in Au+Au collisions Black: Au+Au Red: p+p J.Jin, QM 2006

  19. Cu+Cu gdirect-h+/- vs. p0-h+/- jet yields A. Adare, WWND 2007 per-trigger jet pair yields: systematic from Rg systematic from subtraction method

  20. Opening angle (radians) pT (GeV/c) Distinguishing direct photons from p0 • 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 Opening angle of p0 as a function of pT

  21. Distinguishing direct photons from p0 • Reject 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

  22. Distinguishing single photons from 2 decay photons in STAR Shower-max detector (Dh,DF)~(0.007,0.007) • Hamed, Collective Dynamics 2007

  23. p0 g p0 bin mixed bin photon bin STAR g-jet in p+p Subhasis Chattopadhyay, QM 2006 -- Improved p0/g separation using shower profile in SMD

  24. STAR g-jet in p+p Comparison to HIJING p+p – verification of method Ettrigger from 8.5GeV/c to 10.5 GeV Subhasis Chattopadhyay, QM 2006

  25. Next Steps • Do similar analysis for Au+Au collisions as was done in p+p and d+Au • Challenge within large-multiplicity background • Run-7 results coming soon • Results must be interpreted together with di-jet results and/or within model to take surface bias into account

  26. Summary • Di-Jets’ surface bias  results indicate jets are either transmitted with negligible energy loss or absorbed completely • Photon-Jet Measurement will complement the di-jet for more complete probe • A significant (statistically and systematically) g-jet Measurement in Au+Au is needed • Need a great deal of statistics! • Stay tuned for the results from Run-7…. Increase in statistics over Run-4 Au+Au > factor of 10

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