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

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

3. Why  -jet? q qg γ q q g γ ComptonAnnihilation q q q q g g g γ γ Bremsstrahlung “Golden Probe” of energy loss - calibrated probe of jet energy - X.-N. Wang et al. Gluon jet vs. quark jet energy loss – compare  -jet to  0 -jet (in  -jet, the jet will mostly be from quarks) - need to understand the surface bias to interpret data ! Dominant

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  -jet coincidence

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

6. Single-Particle Spectra – What have we learned? Hadrons are suppressed by factor ~5, Photons are not PHENIX, Phys. Rev. Lett. 96, 202301 (2006) PHENIX  0 Spectrum Fractional “effective” energy loss S loss ~ 20% in most central Au+Au collisions PHENIX preliminary

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

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

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

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

11. What can be learned? Di-jets suffer surface bias of trigger jet, punch-through of away-side (or only tangential emission)  -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  q

12. Challenging Measurement Trigger photon measured with electro- magnetic calorimeter Large background from hadronic decays (  0,  ) Background sources of photons (hadronic decays) suppressed in central Au+Au collisions PHENIX, Phys. Rev. Lett. 94, 232301 (2005)

13.  -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 (  0,  ) Background signal is di-jet (  0 -jet) Subtract from inclusive signal Need large-statistics data set for  -jet measurement

15. Statistical measurement of  -jet yields Measure inclusive  -jet correlation function Measure  0 -jet (  -jet) correlation function Construct resulting  decay -jet correlation taking into account decay kinematics Subtract  decay -jet from  inclusive -jet  Result is  direct -jet

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

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

18. PHENIX  -jet in Au+Au collisions J.Jin, QM 2006 Black: Au+Au Red: p+p 1/N trig dN/d   (rad)

19. Cu+Cu  direct -h +/- vs.   -h +/- jet yields per-trigger jet pair yields: systematic from R  systematic from subtraction method A. Adare, WWND 2007

20. Distinguishing direct photons from  0 p T (GeV/c) Opening angle (radians) Opening angle of  0 as a function of p T Segmentation of calorimeter towers in STAR (  ) = (0.05,0.05)  2 decay photons hit same tower starting p T ~ 5 or 6 GeV/c Segmentation in PHENIX ~ 0.01 radians

21. Distinguishing direct photons from  0 Reject clusters that look like 2 close photons (from  0 decay) from shower shape Retain sample of correlations that are  direct - jet “rich” Estimate remaining contamination from  0 - jet Measure and subtract  0 -jet away-side yield

22. Distinguishing single photons from 2 decay photons in STAR Shower-max detector (  )~(0.007,0.007) A.Hamed, Collective Dynamics 2007

23. STAR  -jet in p+p Subhasis Chattopadhyay, QM 2006 -- Improved  0 /  separation using shower profile in SMD 00   0 bin mixed bin photon bin

24. STAR  -jet in p+p Subhasis Chattopadhyay, QM 2006 E t trigger from 8.5GeV/c to 10.5 GeV Comparison to HIJING p+p – verification of method

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)  - 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