1. Direct Photon Production at RHIC Stefan Bathe UC Riverside University of Frankfurt, May 31, 2007

2. University of Frankfurt, 2007 Stefan Bathe 2 Introduction Leading Particle Direct  Hadrons g q frag. ● Photons don’t interact strongly ● Two great promises: ♦ Initial temperature, transition temperature via thermal photons ♦ Jet energy scale, precise energy loss via  -jet correlations ● Problems ♦ richer probe than originally thought (medium-induced photons) ♦ Old idea of unfolding photon sources (starting from the clearly understood high pT spectra and moving down in p T ) seriously challenged

3. University of Frankfurt, 2007 Stefan Bathe 3 Photon Sources in N+N ● Experimenter’s definition of direct photons ♦ not from hadronic decays ● Elementary processes ♦ compton: ♦ annihilation: ♦ fragmentation = bremsstrahlung LO Compton Annihilation Bremsstrahlung LL

4. University of Frankfurt, 2007 Stefan Bathe 4 Photon Sources in A+A Photons in A+A Direct PhotonsDecay Photons Non-thermalthermalHard+thermal Initial hard scattering Pre-equili- brium photons QGPHadron gas Interaction of hard parton with QGP 1) and 2) Medium induced photon bremsstrahlung pQCD or prompt photons (as N+N, but modified) from medium

5. University of Frankfurt, 2007 Stefan Bathe 5 log t pTpT 11010 7 (GeV) (fm/c) hadron decays hadron gas sQGP jet-thermal hard scatt. jet brems. vs. creation time in principle: working one’s way down from the highest p T (and excluding hadron decays) one might be able to disentangle different sources Disentangling Sources

6. University of Frankfurt, 2007 Stefan Bathe 6 C. Gale, NPA 774, 335 (2006) some chance of disentangling sources (using v 2 and isolation, more later) Possible Scenario C. Gale, NPA 785, 93c (2007) even more difficult

7. University of Frankfurt, 2007 Stefan Bathe 7    PHENIX Experiment ● EMCal ♦  0 via  0  ♦ |  | < 0.35 ♦  = 0.01 x 0.01 Lead scintillator (PbSc) Lead glass (PbGl) ● DC, PC ♦ Charged particles ● RICH ♦ e ± ID ● Muon arms (not shown, forward rapidity)

8. University of Frankfurt, 2007 Stefan Bathe 8 1. High p T inclusive

9. University of Frankfurt, 2007 Stefan Bathe 9 Photon Run-2 Result—The T-Shirt Plot  0 ’s,  ’s suppressed  0 suppression caused by created medium direct photons not suppressed

10. University of Frankfurt, 2007 Stefan Bathe 10 New p+p Run-5 ● agreement with NLO pQCD ● Important baseline for Au+Au

11. University of Frankfurt, 2007 Stefan Bathe 11 Parameterization of p+p Data ● For R AA parameterization of p+p reference used ● While consistent, data tends to be higher than pQCD by at least 20%, increasing to low and high p T Data Fit

12. University of Frankfurt, 2007 Stefan Bathe 12 New Au+Au, Run-4 Phys. Rev. Lett. 94, (2005) 232301

13. University of Frankfurt, 2007 Stefan Bathe 13 Direct Photon R AA First direct photon R AA using p+p data as reference R AA with pQCD reference R AA with p+p data

14. University of Frankfurt, 2007 Stefan Bathe 14 Updated T-Shirt Plot Direct  R AA with measured p+p reference data η π 0 ● Direct photons and  0 touch at highest p T ● Still consistent with ♦ final state effect for  0 and initial state effect for direct photons ♦ structure functions different for the two

15. University of Frankfurt, 2007 Stefan Bathe 15 Comparison to Theory ● Turbide et al. ♦ Jet photons + pQCD + thermal ♦ AMY formalism for jet-quenching of fragmentation photons ♦ Data systematically below theory ♦ Phys. Rev. C72 (2005) 014906 + private communication ● F. Arleo ♦ pQCD photons only ♦ High-p T suppression due to isospin effect, shadowing, and energy loss ♦ BDMPS for jet-quenching ♦ JHEP 0609 (2006) 015

16. University of Frankfurt, 2007 Stefan Bathe 16 Further Comparison Bremsstrahlung photons through bulk matter B.G. Zakharov, JETP Lett. 80 (2004) 1

17. University of Frankfurt, 2007 Stefan Bathe 17 Experimental Handles ● suppression seen in p T region where sensitive to detector bias (cluster merging) ● Use different detectors ♦ So far only PbSc analyzed ♦ PbGl better spatial resolution ♦ Less sensitive to merging ● x T scaling ♦ If suppression due to modified structure function, it should depend on x T, not p T ♦ At lower √s, lower p T for same x T ♦ shower merging smaller at lower p T ♦ But other sources come into play here

18. University of Frankfurt, 2007 Stefan Bathe 18 2. Elliptic Flow

19. University of Frankfurt, 2007 Stefan Bathe 19 Direct Photon Elliptic Flow ● Jet-photons and induced bremsstrahlung photons : negative v 2 for direct photons ● Jet-quenching  less fragmentation photons: positive v 2 for direct photons

20. University of Frankfurt, 2007 Stefan Bathe 20 S.Turbide, C.Gale, R.J.Fries, PRL 96 032303 (2006) R. Chatterjee et al., PRL 96, 202302 (2006) Thermal photons produced throughout expansion history and reflect quark anisotropy: net effect positive v 2 Jet photons, fragmentation photons: Net effect negative v 2 Theoretical Predictions

21. University of Frankfurt, 2007 Stefan Bathe 21 ● Measure inclusive and hadron v 2 by reaction plane method ● Calculate direct photon v 2, using double ratio R, as: R * v 2 inc – v 2 BG R = N inc / N BG v 2 dir = R – 1 Measurement

22. University of Frankfurt, 2007 Stefan Bathe 22 v 2 consistent with zero Systematic errors mostly from R: large background of decay photons at low p T 7 Result

23. University of Frankfurt, 2007 Stefan Bathe 23 Disentangling++ Contributionsofterv2isol. 1 Compton, annihi.0yes 2 jet-thermal<0yes 3 Fragmentation>0no 4 induced. brems.<0no 1 2 3 4 1 2 3 4 NPA 774, 335 (2006)NPA 785, 93c (2007)

24. University of Frankfurt, 2007 Stefan Bathe 24 3.  -jet

25. University of Frankfurt, 2007 Stefan Bathe 25 Photon-tagged Jets  Hadrons Observing jets and dijets through leading hadrons biases toward high fragmentation z, and also toward sources at the periphery. Tagging jets opposite isolated direct photon measures jet p T, and does not bias fragmentation or location of jet production. “Clean” measurement of medium effects on hadronization.

26. University of Frankfurt, 2007 Stefan Bathe 26 Angelis et al. Nucl. Phys. B327 (1989) 541 isolated photon in back-to-back  -jet pair good measure of total jet energy (modulo initial k T ) R110, 62 GeV jet E-scale in p+p at CERN-ISR disappearance of  -side partners with p T >1.0 GeV→ bremsstrahlung small

27. University of Frankfurt, 2007 Stefan Bathe 27 Correlation Function  Trigger Assoc Mixed events – correct for PHENIX pair acceptance

28. University of Frankfurt, 2007 Stefan Bathe 28  -Jet Correlations  Inclusive  -h  Decay  -h contribution (via  0 -hadron)  Direct  -h ! p+p collisions at 200 GeV

29. University of Frankfurt, 2007 Stefan Bathe 29 Comparison to Pythia

30. University of Frankfurt, 2007 Stefan Bathe 30  -Jet Correlations in AuAu

31. University of Frankfurt, 2007 Stefan Bathe 31 systematic from R  systematic from subtraction method PHENIX Cu+Cu – jet E-scale

32. University of Frankfurt, 2007 Stefan Bathe 32 3. Low p T

33. University of Frankfurt, 2007 Stefan Bathe 33 A new technique: opening up the phase space M inv pTpT direct photon analysis new dilepton analysis conventional dilepton analysis 0

34. University of Frankfurt, 2007 Stefan Bathe 34 phase space factorform factor invariant mass of virtual photon invariant mass of Dalitz pair phase space factorform factor invariant mass of Dalitz pair invariant mass of virtual photon The Idea ● Start from Dalitz decay ● Calculate invariant mass distribution of Dalitz pairs ● Now direct photons ● Any source of real  produces virtual  with very low mass ● Rate and mass distribution given by same formula ♦ No phase space factor for m ee << p T photon 00   00  e+e+ e-e-  Compton q  g q q  g q e+e+ e-e-

35. University of Frankfurt, 2007 Stefan Bathe 35 ● Calculate ratios of various M inv bins to lowest one: R data ● If no direct photons: ratios correspond to Dalitz decays ● If excess: direct photons In Practice ÷ ÷ ÷ 0-30 90-140 140-200 MeV 200-300 R data ● Material conversion pairs removed by analysis cut ● Combinatorics removed by mixed events

36. University of Frankfurt, 2007 Stefan Bathe 36

37. University of Frankfurt, 2007 Stefan Bathe 37   

38. University of Frankfurt, 2007 Stefan Bathe 38    S/B=~1   

39. University of Frankfurt, 2007 Stefan Bathe 39    S/B=~1    RR RR R direct calculated from Dalitz formula measured R data ÷

40. University of Frankfurt, 2007 Stefan Bathe 40    S/B=~1    calculated from Dalitz formula measured R data ÷ RR RR R direct

41. University of Frankfurt, 2007 Stefan Bathe 41    S/B=~1    calculated from Dalitz formula measured R data ÷ RR RR R direct measured with EMCal Here we are… ~25 % systematic error : ~20 % from measured  0 ratio ~10 % from  inclusive ~5 % acceptance

42. University of Frankfurt, 2007 Stefan Bathe 42 Comparison to Conventional result ( + 1 )

43. University of Frankfurt, 2007 Stefan Bathe 43 The Spectrum Compare to published Run2 result: PRL94 232301

44. University of Frankfurt, 2007 Stefan Bathe 44  direct ● preliminary direct photon spectrum at 1-5 GeV/c from  *

45. University of Frankfurt, 2007 Stefan Bathe 45 The Spectrum Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) above (questionable) pQCD

46. University of Frankfurt, 2007 Stefan Bathe 46 The Spectrum Compare to thermal model 2+1 hydro T 0 ave =360 MeV(T 0 max =570 MeV)  0 =0.15 fm/c D. d’Enterria, D. Perresounko nucl-th/0503054 Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) data above thermal at high p T above (questionable) pQCD

47. University of Frankfurt, 2007 Stefan Bathe 47 The Spectrum Compare to thermal + pQCD Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) data consistent with thermal + pQCD above (questionable) pQCD Compare to thermal model 2+1 hydro T 0 ave =360 MeV(T 0 max =570 MeV)  0 =0.15 fm/c D. d’Enterria, D. Perresounko nucl-th/0503054 data above thermal at high p T

48. University of Frankfurt, 2007 Stefan Bathe 48 The Spectrum Compare to thermal + pQCD Compare to NLO pQCD L.E.Gordon and W. Vogelsang Phys. Rev. D48, 3136 (1993) data consistent with thermal + pQCD above (questionable) pQCD Compare to thermal model 2+1 hydro T 0 ave =360 MeV(T 0 max =570 MeV)  0 =0.15 fm/c D. d’Enterria, D. Perresounko nucl-th/0503054 data above thermal at high p T Needs confirmation from p+p measurement

49. University of Frankfurt, 2007 Stefan Bathe 49 More Comparisons

50. University of Frankfurt, 2007 Stefan Bathe 50 Low p T  new conversion method Direct Photons, Au+Au at 200 GeV External conversions Internal Conversions o New measurement with external conversions o Improved systematics o Confirmation of previous results

51. University of Frankfurt, 2007 Stefan Bathe 51 Internal conversion: d+Au Fit measured mass spectrum with function: a - absolute normalization b – fraction of direct photons:

52. University of Frankfurt, 2007 Stefan Bathe 52 Comparison: d+Au--Au+Au N coll -scaled d+Au agrees with Au+Au =>Systematic errors too large to extract thermal photons =>but consistent with thermal photons

53. University of Frankfurt, 2007 Stefan Bathe 53 Outlook ● Current (2007) and future (2008) higher statistics (x4, x10) runs ♦ will improve inclusive, v 2, and  -jet measurement ● Addition of RxNP detector (2007) ♦ will improve v 2 measurement (x2 resolution) ♦ thereby facilitate disentangling of sources ● Isolation cut (possible at high p T with high statistics) ♦ will improve disentangling of sources ♦ give independent systematics for high p T inclusive and  -jet ● Future high-statistics p+A run ♦ will allow precision measurement of structure function in nucleus ♦ thereby eliminate ambiguity as to initial vs. final state modifications of spectrum ● Low p T measurement in p+p ♦ Will provide reference for Au+Au measurement ● New techniques (e.g.  HBT) ♦ Will give independent systemeatics for low p T measurement