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Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC

Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC. Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration. Physics in Heavy Ion Collisions (HIC) Why Quark Gluon Plasma (QGP) ?. Our dream Dream should come tru e

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Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC

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  1. Electromagnetic emission from hot medium measured by the PHENIX experiment at RHIC Takao Sakaguchi Brookhaven National Laboratory For the PHENIX Collaboration

  2. Physics in Heavy Ion Collisions (HIC)Why Quark Gluon Plasma (QGP) ? • Our dream • Dream should come true • Fundamental and thorough understanding of QCD • QCD is a theory to describe strong interaction • Questions have been: • Quark confinement • Origin of proton (hadron) Mass • Both questions rely on low Q2 region, where as(Q2) is not small • QCD in thermal region • New at RHIC: Probing the non-pQCD matter by pQCD matter T. Sakaguchi, Physics Colloquium

  3. Electromagnetic probe (photon) basics Photon Production: Yield  s • 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) e+ g* e- g T. Sakaguchi, Rutherford Conf., Manchester, UK

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

  5. Possible sources of photons 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, Rutherford Conf., Manchester, UK

  6. Possible sources of photons 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, Rutherford Conf., Manchester, UK

  7. 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, Rutherford Conf., Manchester, UK

  8. 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, Rutherford Conf., Manchester, UK

  9. Profiling collision systems using photons 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 Once the path length dependence has been fixed, system expansion scenario can be studied by looking at photons at different rapidities. e.g. PRC71 (2005) 064905 jet fragment photon annihilation compton scattering jet v2 > 0 Bremsstrahlung (energy loss) v2 < 0 g g For prompt photons: v2~0 T. Sakaguchi, Rutherford Conf., Manchester, UK

  10. Hadron decay photon v2 subtracted from inclusive photon v2. Low pT photon flow Au+Au@200 GeV minimum bias arXiv:1105.4126 Au+Au@200 GeV minimum bias • p0v2 Direct photons • inclusive photons • (thermal are ~10%) T. Sakaguchi, Rutherford Conf., Manchester, UK

  11. Later thermalization gives larger v2 (QGP photons) Large photon flow is not explained by models What we learn from model comparison Hydro after t0 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, SrivastavaPRC79, 021901 (2009) T. Sakaguchi, Rutherford Conf., Manchester, UK

  12. Direct photons are a power tool to investigate the collision dynamics From initial state till the freezeout of the system Hard scattering photons have been measured in nucleus collisions for the first time Low pT photons show thermal characteristics (exponential slope) Direct photon v2 has been measured for the first time Powerful source detector Unexpectedly large flow was seen. Not explainable by models Summary T. Sakaguchi, Rutherford Conf., Manchester, UK

  13. Backup T. Sakaguchi, Rutherford Conf., Manchester, UK

  14. 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 much smaller than one in pi0 Within quoted errors, the effect is same for pi0 and photon production Recombination model needs improvement Initial kT or recombination? Pi0 RAA in d+Au at 200GeV. PRL91, 072303 (2003) T. Sakaguchi, Rutherford Conf., Manchester, UK

  15. 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, Rutherford Conf., Manchester, UK

  16. γ e- e+ e+ π0 e+ e- π0 π0 γ e- γ Analysis Like-sign • Reconstruct Mass and pT of e+e- • Identify and reject conversion photons in beam pipe, using angular correlation of electron pairs • Subtract combinatorial background • Background checked by like-sign dist. • Apply efficiency correction • Subtract additional correlated background: • Back-to-back jet contribution • Determine amount in like-sign dist, and apply to unlike-sign dist. • Compare with known hadronic sources Unlike-sign PRC81, 034911(2010), arXiv:0912.0244 T. Sakaguchi, Rutherford Conf., Manchester, UK

  17. OutcomefromAu+Au collisions PRC81, 034911(2010), arXiv:0912.0244 T. Sakaguchi, Rutherford Conf., Manchester, UK • 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

  18. 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, Rutherford Conf., Manchester, UK

  19. Theory prediction on dilepton/photons Photons 1/M g*ee Dileptons qqg*e+e- ≈(M2e-E/T)×1/M PRC 69(2004)014903 Ralf Rapp, priv. comm. T. Sakaguchi, Rutherford Conf., Manchester, UK Similar source are seen in both dileptons and photons Internal conversion of photons is not shown in dilepton calculation

  20. g p DC e+ e- PC1 magnetic field & tracking detectors PC3 Dilepton measurement in PHENIX 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, Rutherford Conf., Manchester, UK

  21. 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, Rutherford Conf., Manchester, UK

  22. Shown are in p+p, d+Au and Au+Au collisions at √sNN=200GeV Lines are NLO pQCD calculation with mass scales (pT=0.5, 1.0, 2.0) Excess in min. bias Au+Au is much higher than that in d+Au Direct g to inclusive g ratios T. Sakaguchi, Rutherford Conf., Manchester, UK

  23. We will look at these regions at RHIC Hard region Perturbative QCD works Thermal region Perturbative QCD doesn’t work • Interaction is non-perturbative in the low energy region • Hard to understand  QGP is a strongly interacting lab at various as T. Sakaguchi, Physics Colloquium

  24. Time evolution of heavy ion collisions Gluon Plasma QGP phase Mixed phase Hadronization + Expansion • Gold ions almost “pass through” each other • Mid-rapidity region is full of small-x gluons • Turns into Gluon plasma • Gluon  quark + anti-quark  QGP • Cooling QGP  Mixed phase  Hadronic stage • Temperature expected: T=160-190MeV (Threshold) Petreczky, QM2009 T. Sakaguchi, Physics Colloquium

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