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Photon-Jet Correlations at RHIC

Photon-Jet Correlations at RHIC. Saskia Mioduszewski Texas A&M University. 18 July, 2007. Outline. What can be learned from g -jet measurements? How to measure? Current Status of Measurement at RHIC. Dominant. g. q. γ. q. g. γ. q. g. q. Bremsstrahlung. Why g -jet?. q.

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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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