1. Direct Photons: Flow, Thermal Yield and High p T R AA Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

2. Direct photons basics Production Process – Compton and annihilation (LO, direct) – Fragmentation (NLO) – Escape the system unscathed Carry dynamical information of the state Temperature, Degrees of freedom – Immune from hadronization (fragmentation) process at leading order – Initial state nuclear effect Cronin effect (k T broardening) 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 2 Photon Production: Yield   s   e+e+ e-e-

3. PHENIX detector 2 central arms: electrons, photons, hadrons – charmonium J/ ,  ’ -> e + e - – vector meson r, w,  -> e + e - – high p T p o, p +, p - – direct photons – open charm – hadron physics Au-Au & p-p spin PC1 PC3 DC magnetic field & tracking detectors e+e+ ee   Designed to measure rare probes: + high rate capability & granularity + good mass resolution and particle ID - limited acceptance 2012-04-10 3 T. Sakaguchi, WWND2012@Puerto Rico

4. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 4 High pT  dir in p+p – (p)QCD test NLO pQCD calculation of  dir yield is tested with p+p collisions The calculation works very well Aurenche et al., PRD73, 094007(2007)

5. Scaling in p+p direct photons Plotting cross-sections from various experiments against x T = 2p T /√s Hard scattering process should scale with x T Scale yields by (√s) n PHENIX data includes virtual photon results in low p T (1<p T <5GeV/c) n=4.5 makes a universal line – Consistent with expectation from NLO pQCD. – See similar analysis arXiv:1202.1762 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 5

6. 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 6 A theory: F. Arleo (JHEP 0609 (2006) 015) Isospin effect, in addition to jet-quenching(BDMPS) and shadowing. Jet-photon conversion is not taken into account Low pT region is underestimated because of lack of jet-photon conversion? Direct photons in 200GeV Au+Au Direct photons suppressed at very high pT? – Final result is coming

7. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 7 (fm/c) log t 1 10 10 7 hadron decays sQGP hard scatt Rich sources of photons in QGP jet Brems. jet-thermal parton-medium interaction hadron gas EE Rate Hadron Gas sQGP Jet-Thermal Jet Brems. Hard Scatt See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005)

8. Adding virtuality in photon measurement 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 8 (fm/c) log t 1 10 10 7 hadron decays hadron gas sQGP hard scatt Mass (GeV/c 2 ) 0.5 1  *  e+e- virtuality jet Brems. jet-thermal parton-medium interaction By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c 2 )

9. 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 9 Compton q  g q e+e+ e-e- Internal conv. One parameter fit: (1-r)f c + r f d f c : cocktail calc., f d : direct photon calc. Focus on the mass region where  0 contribution dies out For M<<p T and M<300MeV/c 2 – qq ->  * contribution is small – Mainly from internal conversion of photons Can be converted to real photon yield using Kroll-Wada formula – Known as the formula for Dalitz decay spectra PRL104,132301(2010), arXiv:0804.4168 Low p T photons with very small mass

10. System size dependence of  fraction  fraction = Yield direct / Yield inclusive Lines are NLO pQCD calculation with mass scales (pT=0.5, 1.0, 2.0) Largest excess above pQCD is seen at Au+Au. – Moderately in Cu+Cu. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 10 No excess in d+Au (no medium) Excess also in Cu+Cu

11. d+Au Min. Bias Low p T photons in Au+Au (thermal?) 11 PRL104,132301(2010), arXiv:0804.4168 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico Inclusive photon ×  dir /  inc Fitted the spectra with p+p fit + exponential function – T ave = 221  19 stat  19 syst MeV (Minimum Bias) Nuclear effect measured in d+Au does not explain the photons in Au+Au Au+Au

12. Initial k T broadening or recombination? 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 12 Recombination model claims that the Cronin effect in hadron production is built up by recombination – e.g. R. Hwa, Eur.Phys.J.C43:233(2005) – Cronin effect in direct photon production should be smaller than one in  0 Within quoted errors, the effect is same for  0 and photon production   RAA in d+Au at 200GeV. PRL91, 072303 (2003)

13. Comparison with a model calculation Nuclear Effect is slightly seen. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 13

14. Direct photon v 2 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 14

15. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 15 Photon source detector ~v2~ annihilation compton scattering Bremsstrahlung (energy loss) jet jet fragment photon v 2 > 0 v 2 < 0 Depending the process of photon production, path length dependence of direct photon yield varies – v 2 of the direct photons will become a source detector – Later thermalization gives larger v 2 For prompt photons: v 2 ~0 Turbide et al., PRC77, 024909 (2008)

16. Measuring direct photon v 2 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 16 Calculation of direct photon v 2 = inclusive photon v 2 - background photon v 2 (    etc) PHENIX, arXiv:1105.4126

17. Centrality dependence of direct photon v 2 Very large flow in low pT v 2 goes to 0 at high p T – Hard scattered photons dominate 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 17 PHENIX, arXiv:1105.4126

18. Comparison with models. No success.. Later thermalization gives larger v 2 (QGP photons) Large photon flow is not explained by partonic flow models 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 18 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, Srivastava PRC79, 021901 (2009) Hydro after  0

19. This fits to data well, but.. Large flow can not be produced in partonic phase, but could be in hadron gas phase This model changed ingredients of photon spectra drastically! – We realized the importance of the data… 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 19 thermal + prim.  van Hees, Gale, Rapp, PRC84, 054906 (2011)

20. Summary 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 20 Primordial hard scattering process is investigated with direct photons in p+p collisions – x T scaling parameter n=4.5 makes a universal curve Low p T photons in Au+Au show thermal characteristics (exponential slope) Low p T photons in d+Au exhibit initial state nuclear effect – The effect is very small. Direct photon v 2 has been measured for the first time – Powerful source detector – Unexpectedly large flow was seen. Not explainable by models except for the one assuming long hadron gas phase.

21. Backup 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 21

22. 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 22 New production mechanism introduced Jet in-medium bremsstrahlung Jet-photon conversion Both are “thermal  hard” Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) Compton scattering of hard scattered and thermal partons (Jet-photon conversion) Turbide et al., PRC72, 014906 (2005) R. Fries et al., PRC72, 041902 (2005) Turbide et al., PRC77, 024909 (2008) Liu et al., arXiv:0712.3619, etc..

23. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 23 A plate ~After cooking up ingredients~

24. 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 24 Is it (only) an isospin effect? Taking for example, the isospin effect: Direct photon cross-sections for p+p, p+n and n+n are different because of different charge contents (  e q 2 ) Effect can be estimated from NLO pQCD calclation of p+p, p+n and n+n – In low pT, quarks are from gluon split  no difference between n and p – At high pT, contribution of constituent quarks manifests Minimum bias Au+Au can be calculated by: (  AA /N coll )/  pp vs pT (  AA /N coll )/  pp vs xT Same suppression will be seen in lower pT at  s NN =62.4GeV TS, INPC07, arXiv.org:0708.4265

25. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 25 Looks like there is an isospin effect (and/or PDF effect) Question: p+p is a right reference to take? – Isospin effect is electric charge dependent, which affects to photons;  0 is color charge dependent – Therefore, e-loss models so far are still valid Also see: Miki, Session XV The test: 62GeV Au+Au direct photons ~18GeV/c@200GeV Both are reasonable!

26. Calculations reasonably agree with data Factors of two to be worked on.. Correlation between T and   T ini = 300 to 600 MeV   = 0.15 to 0.5 fm/c 26 T. Sakaguchi, WWND2012@Puerto Rico 2012-04-10 0-20% Au+Au PRL104,132301(2010), arXiv:0804.4168

27. 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 27 CentdN/dy (pT>1GeV) Slope (MeV)  2 /D OF 0-20%1.50±0.23± 0.35 221±19± 19 4.7/4 20-40%0.65±0.08± 0.15 217±18± 16 5.0/3 MinBias0.49±0.05± 0.11 233±14± 19 3.2/4 Inclusive photon ×  dir /  inc Fitted the spectra with p+p fit + exponential function Barely dependent of centrality PRL104,132301(2010), arXiv:0804.4168 Direct photons through dileptons

28. Reconstruct Mass and pT of e+e- – Same as real photons – Identify conversion photons in beam pipe using their orientation w.r.t. the magnetic field Reject them –   e+e- at r≠0 have m≠0 (artifact of PHENIX tracking: no tracking before the field) 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 28 Dilepton analysis (I) z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay Compton q  g q e+e+ e-e- e+e+ e-e- External conv. Internal conv.

29. PHENIX applied internal conversion technique – Real photons can convert to virtual photons – Inv. mass shapes for Dalitz decay of mesons are calculable using Kroll-Wada formula If M<<pT, the ratio of observed inv. mass to expected is proportional to direct photon excess ratio Take ratio where  0 contribution is small  S/B increases 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 29 Review low-mid pT photons ÷ ÷ ÷ 0-30 90-140 140-200 200-300 R data Compton q  g q e+e+ e-e-

30. 2012-04-10T. Sakaguchi, WWND2012@Puerto Rico 30 Hard scattering  dir in Au+Au (high p T ) Blue line: N coll scaled p+p cross-section Au+Au = p+p x T AB holds – pQCD factorization works NLO pQCD works.  Non-pert. QCD may work in Au+Au system

31. 2012-04-10 31 T. Sakaguchi, WWND2012@Puerto Rico Outcome from Au+Au collisions Comparing with various sources of electron pairs Cocktail of the sources are calculated based on  0 /  spectra measured in PHENIX Huge excess over cocktail calculation is seen in 0.2-0.8GeV/c 2 PRC81, 034911(2010), arXiv:0912.0244

32. Thermal radiation from QGP (1<pT<3GeV) – S/B is ~5-10% – Spectrum is exponential. One can extract temperature, dof, etc.. Hadron-gas interaction (pT<1GeV/c):  (  )   (  ),  K*  K  2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 32 Difficult objects! Photons from QGP ~big challenge~ f B : Bose dist.  em : photon self energy photons dileptons Interesting, but S/B is small 5432154321 S/B ratio

33. Reconstruct Mass and pT of e+e- – Same as real photons – Identify conversion photons in beam pipe using and reject them Subtract combinatorial background Apply efficiency correction Subtract additional correlated background: – Back-to-back jet contribution – well understood from MC Compare with known hadronic sources 2012-04-10 T. Sakaguchi, WWND2012@Puerto Rico 33 Dilepton Analysis π0π0 π0π0 e+e+ e-e- e+e+ e-e- γ γ π0π0 e-e- γ e+e+