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Direct Photons: Flow, Thermal Yield and High p T R AA

Direct Photons: Flow, Thermal Yield and High p T R AA. Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration. Direct photons basics. Photon Production: Yield   s. e +. g*. e -. g. Production Process Compton and annihilation (LO, direct) Fragmentation (NLO)

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Direct Photons: Flow, Thermal Yield and High p T R AA

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  1. Direct Photons: Flow, Thermal Yield and High pT RAA Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

  2. Direct photons basics Photon Production: Yield  s e+ g* e- g • 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 (kTbroardening) T. Sakaguchi, WWND2012@Puerto Rico

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

  4. High pTgdir in p+p – (p)QCD test • NLO pQCD calculation of gdir yield is tested with p+p collisions • The calculation works very well Aurenche et al., PRD73, 094007(2007) T. Sakaguchi, WWND2012@Puerto Rico

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

  6. Direct photons in 200GeV Au+Au • 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 suppressed at very high pT? • Final result is coming T. Sakaguchi, WWND2012@Puerto Rico

  7. Rich sources of photons in QGP See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005) Rate hard scatt HadronGas jet Brems. parton-medium interaction jet-thermal sQGP sQGP hadron gas hadron decays Jet-Thermal Jet Brems. log t 1 107 10 (fm/c) Eg Hard Scatt T. Sakaguchi, WWND2012@Puerto Rico

  8. Adding virtuality in photon measurement hard scatt jet Brems. parton-medium interaction jet-thermal sQGP hadron gas hadron decays g*  e+e- virtuality log t 1 107 0.5 10 (fm/c) 1 By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2) Mass (GeV/c2) T. Sakaguchi, WWND2012@Puerto Rico

  9. Focus on the mass region where p0 contribution dies out For M<<pT and M<300MeV/c2 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 Low pT photons with very small mass e+ Compton e- g* q g q PRL104,132301(2010), arXiv:0804.4168 One parameter fit: (1-r)fc + rfd fc: cocktail calc., fd: direct photon calc. Internal conv. T. Sakaguchi, WWND2012@Puerto Rico

  10. System size dependence of g fraction Excess also in Cu+Cu No excess in d+Au (no medium) • g fraction = Yielddirect / Yieldinclusive • 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. T. Sakaguchi, WWND2012@Puerto Rico

  11. Inclusive photon × gdir/ginc Fitted the spectra with p+p fit + exponential function Tave = 221  19stat  19syst MeV (Minimum Bias) Nuclear effect measured in d+Au does not explain the photons in Au+Au Low pTphotons in Au+Au (thermal?) PRL104,132301(2010), arXiv:0804.4168 Au+Au d+AuMin. Bias 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 p0 Within quoted errors, the effect is same for p0 and photon production Initial kT broadening or recombination? p0 RAA in d+Au at 200GeV. PRL91, 072303 (2003) T. Sakaguchi, WWND2012@Puerto Rico

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

  14. Direct photon v2 T. Sakaguchi, WWND2012@Puerto Rico

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

  16. Measuring direct photon v2 Calculation of direct photon v2 = inclusive photon v2 - background photon v2(p0,h, etc) PHENIX, arXiv:1105.4126 T. Sakaguchi, WWND2012@Puerto Rico

  17. Centrality dependence of direct photon v2 PHENIX, arXiv:1105.4126 • Very large flow in low pT • v2 goes to 0 at high pT • Hard scattered photons dominate T. Sakaguchi, WWND2012@Puerto Rico

  18. Comparison with models. No success.. Hydro after t0 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, SrivastavaPRC79, 021901 (2009) • Later thermalization gives larger v2 (QGP photons) • Large photon flow is not explained by partonic flow models T. Sakaguchi, WWND2012@Puerto Rico

  19. This fits to data well, but.. thermal + prim. g van Hees, Gale, Rapp, PRC84, 054906 (2011) • 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… T. Sakaguchi, WWND2012@Puerto Rico

  20. Primordial hard scattering process is investigated with direct photons in p+p collisions xT scaling parameter n=4.5 makes a universal curve Low pT photons in Au+Aushow thermal characteristics (exponential slope) Low pT photons in d+Au exhibit initial state nuclear effect The effect is very small. Direct photon v2 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. Summary T. Sakaguchi, WWND2012@Puerto Rico

  21. Backup T. Sakaguchi, WWND2012@Puerto Rico

  22. New production mechanism introduced Jet-photon conversion Jet in-medium bremsstrahlung • Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) • Compton scattering of hard scattered and thermal partons (Jet-photon conversion) Both are “thermal  hard” 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.. T. Sakaguchi, WWND2012@Puerto Rico

  23. A plate ~After cooking up ingredients~ 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 (  eq2) • 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: (sAA/Ncoll)/spp vs pT (sAA/Ncoll)/spp vs xT Same suppression will be seen in lower pT at sNN=62.4GeV T. Sakaguchi, WWND2012@Puerto Rico TS, INPC07, arXiv.org:0708.4265

  25. Both are reasonable! The test: 62GeV Au+Au direct photons • 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 ~18GeV/c@200GeV Also see: Miki, Session XV T. Sakaguchi, WWND2012@Puerto Rico

  26. Calculations reasonably agree with data 0-20% Au+Au Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c PRL104,132301(2010), arXiv:0804.4168 • Factors of two to be worked on .. • Correlation between T and t0 T. Sakaguchi, WWND2012@Puerto Rico

  27. Inclusive photon × gdir/ginc Fitted the spectra with p+p fit + exponential function Barely dependent of centrality Direct photons through dileptons PRL104,132301(2010), arXiv:0804.4168 T. Sakaguchi, WWND2012@Puerto Rico

  28. Conversion pair Dalitz decay e- z z B B y y e- e+ x x e+ Internal conv. External conv. e+ e+ e- Compton e- g* q g q Dilepton analysis (I) T. Sakaguchi, WWND2012@Puerto Rico • 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 • ge+e- at r≠0 have m≠0(artifact of PHENIX tracking: no tracking before the field)

  29. 0-30 90-140 200-300 140-200 Rdata ÷ ÷ e+ Compton e- g* ÷ q g q Review low-mid pT photons T. Sakaguchi, WWND2012@Puerto Rico • 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 p0 contribution is small  S/B increases

  30. Blue line: Ncoll scaled p+p cross-section Hard scattering gdir in Au+Au (high pT) T. Sakaguchi, WWND2012@Puerto Rico • Au+Au = p+px TAB holds – pQCD factorization works • NLO pQCD works. Non-pert. QCD may work in Au+Au system

  31. OutcomefromAu+Au collisions PRC81, 034911(2010), arXiv:0912.0244 T. Sakaguchi, WWND2012@Puerto Rico • Comparing with various sources of electron pairs • Cocktail of the sources are calculated based on p0/h spectra measured in PHENIX • Huge excess over cocktail calculation is seen in 0.2-0.8GeV/c2

  32. photons dileptons fB: Bose dist. em: photon self energy Difficult objects! Photons from QGP~big challenge~ Interesting, but S/B is small S/B ratio 5 4 3 2 1 • 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 T. Sakaguchi, WWND2012@Puerto Rico

  33. γ e- e+ e+ π0 e+ e- π0 π0 γ e- γ Dilepton Analysis • 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 T. Sakaguchi, WWND2012@Puerto Rico

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