Photons at RHIC Henner Büsching FIAS – University of Frankfurt Jyväskylä - March 2007

2 In a perfect world… RHIC: 200 GeV pp Run over RHIC: 200 GeV Run Started Hoax! News: RHIC is running!

3 …temperature of the QGP can be measured by thermal photon radiation… …photons don’t interact with the medium… …good probe of the medium… … gamma- jet correlations… XXX’s Diploma Thesis …in a simple world…

4 …in the real world… PHENIX Experiment: Problem to extract Large uncertainties Theory: Do we understand all sources?  measured /  background    measured / (    background Difficult measurement ! Experiment: Need statistics Understand detector? Theory: Do we understand all sources?

5 In the real world…a wide field! PHENIX Direct  v 2  -jet Shapes Virtual  Intermediate p T Studies Statistical Method Ext. Conversions Photon Tagging Reaction Plane Dependence Thermal  Bremsstrahlung  Jet Medium Interactions Isolation Cut Precision!

Status So far unfulfilled: Initial temperature via thermal photons Energy loss via g-jet correlations

7 Run4 data Direct photon spectrum for 1 < p T < 4.5 GeV via internal conversion Punchline first: Where we are 1 Low p T

Punchline first: Where we are 2 Run2 data High p T

Punchline first: Where we are 3 Run4 data High p T - Proton Reference

Who ordered all these photons?

11 Compton Annihilation Hard direct photons: Direct component q g q  q q _  g photons in A+A direct photonsdecay photons hard direct fragmentation bremsstrahlung Photon Sources in Au+Au Un-affected by medium

12 Hard direct photons: bremsstrahlung and fragmentation Affected by medium q g g q  q g  q photons in A+A direct photonsdecay photons hard direct fragmentation bremsstrahlung Photon Sources in Au+Au

13 photons in A+A direct photonsdecay photons hardthermaljet-plasma-interact. QGPhadron gasdirectfragmentation pre-equilibrium jet-  - conv. medium-induc.  bremsstr. Photon Sources in Au+Au

14 Jet-plasma interaction jet-photon- conversion photons in A+A direct photonsdecay photons hardthermaljet-plasma-interact. QGPhadron gasdirectfragmentation pre-equilibrium jet-  - conv. medium-induc.  bremsstr. Photon Sources in Au+Au

15 Jet-plasma interaction: medium-induced bremsstrahlung q QGP photons in A+A direct photonsdecay photons hardthermaljet-plasma-interact. QGPhadron gasdirectfragmentation pre-equilibrium jet-  - conv. medium-induc.  bremsstr. Photon Sources in Au+Au

16 photons in A+A direct photonsdecay photons hardthermaljet-plasma-interact. QGPhadron gasdirectfragmentation pre-equilibrium jet-  - conv. medium-induc.  bremsstr. Photon Sources in Au+Au Created in all phases of the collision Once created, they survive But this makes measurements hard to interpret

17 Spectra - Schematic Thermal photons relevant below p T ~ 3 GeV/c Hard Photons for p T > ~ 6 GeV/c hard: thermal: Decay photons (  0 → ,  → , …) Jet-Plasma-Konv.

18 C. Gale, hep-ph/ Spectra – Recent Theory

Let’s measure

p+p 00 STAR: Welcome to the Club 2003 d+Au 00 

21 Get clean inclusive photon sample Measure p T spectrum of   and  mesons with high accuracy Calculate number of decay photons per   –Done with Monte-Carlo –m T scaling for (  ),  ’, , … Finally: Subtract decay background from inclusive photon spectrum The traditional way /0/0 Subtraction

22 Inclusive Photon Spectrum Fraction of charged clusters Fraction of neutral background (neutron, anti-neutrons) efficiency acceptance photon conversion Subtraction

23 Uncertainties Many systematic uncertainties of   and photon measurements are highly correlated! Subtraction

24 New ways I Advantage –Good momentum resolution for low p T charged particles (e  e  ) allows photon measurement at low p T –Less trouble with background from hadrons Disadvantage –Radiator needs to be thin: low photon detection probability –Necessary to separate conversion pairs from  0 and  Dalitz decays (  0   e  +e  ) Conversion

25 Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product Correct direct photon candidate sample for contribution from   decay photons with missing partner and for direct photons with fake partners Subtract contribution from ,  etc. decays Tagging New ways II In central A+A collisions the number of random associations is too high for this approach Can work at high p T

26 Results in Au+Au

27 Direct photons in d+Au It works…

28 Internal Conversion different m inv -distribution Any source of real  produces virtual  with very low mass m inv distribution depends on process Rate and mass distribution given by same formula

29 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 Internal Conversion

“Thermal” Discussion of results

31 Low pT – internal conversion 2+1 hydro, d’Enterria, Perresounko Good p+p is missing Ti ~ 300 – 400 MeV

32 Int. Conversions: d+Au Not as successful as in Au+Au: High background in Run3

33 Comparison: d+Au & Au+Au We still have to wait for Run5 p+p

p+p and d+Au

35 Direct photons in p+p Run 3 Run 5 Phys. Rev. Lett. 98, (2007)

36 Direct photons in p+p

37 Initial state: d+Au No indication for nuclear effects NLO pQCD from W.Vogelsang Run 3

High pT – Au+Au

39 R AA at high p T Energy loss No energy loss Reference for photons: pQCD

40 R AA at high p T Reference for photons: measurement

41 R AA with pQCD R AA with p+p data R AA at high p T

42 R AA – Centrality Dependence

43 Scenario: Isospin Shadowing Isospin (p+n vs. p+p) Energy loss: Suppression of direct photons from fragmentation/bremsstrahlung BDMPS No medium induced jet-photon conv. From fragmentation: ~ 30% at p T = 15 GeV/c F. Arleo, JHEP09 (2006) O15 direct fragment

44 Scenario: Isospin II No problem? “primordial” photons unaltered? “medium-induced” photons can be enhanced or suppressed Help from 62 GeV? Lower p T Unclear effects? Jet-  conversion Vogelsang

45 Scenario: Jet-  -Conversion + Energy loss AMY formalism for jet-quenching effect for fragmentation photons. Systematically data points are below theoretical prediction. Turbide et al. Phys. Rev. C72 (2005) Private communication

isolation

47 Isolation Cut Expectation for direct photons:  N+N:isolated  Jet-Photon conversion:isolated  Jet-fragmentation: non isolated  Bremsstrahlung:non isolated Jet  hep-ex/

Flow

49 Direct-Photon v 2  reaction plane Spatial anisotropy Momentum anisotropy v 2 > 0 for hadrons Expectation for direct photons:  N+N:v 2 > 0  Jet-fragmentation: v 2 > 0  Jet-Photon conversion:v 2 < 0  In-medium bremsstr.:v 2 < 0  Net effect:v 2 < 0

50 Expectation for v 2 : negative, |v 2 |  3% − 5% Direct Photon v 2 Turbide, Gale, Fries, Phys.Rev.Lett.96:032303,2006 Here: Assuming no energy loss: jet-g conversion dominates v 2 : v 2 <0

51 PHENIX Preliminary  s NN =200GeV Au+Au Direct Photon v2

52 PHENIX Preliminary  s NN =200GeV Au+Au Direct Photon v2 Run7: PHENIX new Reaction Plane Detector

53 Statistical errors only. Direct Photon v2

54  hadrons The ‘royal’ analysis

55  Inclusive  -h  Decay  -h contribution (via  0 -hadron)  Direct  -h ! p+p at 200 GeV  -jet

56  -jet First results in p+p Consistent with expectations from Pythia First results in Au+Au

57 Summary Direct photons are a difficult measurement –New analysis techniques –STAR started contributing High pT reach in p+p: agrees with pQCD within uncertainty. Direct photons in 200GeV Au+Au: spectra up to 20GeV/c. First direct photons R AA in 200GeV Au+Au collisions with p+p reference. –R AA seems to be below one in very high-p T region –Isospin? –Strong Jet-Medium enhancement ruled out First results on direct photon v2 (v2>0 ?)

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59 Systematic error on direct photon spectra in AuAu 200GeV

60 Systematic error on direct photon spectra in p+p 200GeV

61 More direct photon sources direct photon R AA Bremsstrahlung photons through bulk matter B.G. Zakharov, JETP Lett. 80 (2004) 1