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Measurements of low pT direct photons in PHENIX

Measurements of low pT direct photons in PHENIX. Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo. hard:. Introduction. Quark Gluon Plasma De-confined phase of quarks and gluons Experimental approach at RHIC √s NN = 200GeV Au+Au collisions

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Measurements of low pT direct photons in PHENIX

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  1. Measurements of low pT direct photons in PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo WWND 2008 @ South Padre Island

  2. hard: Introduction • Quark Gluon Plasma • De-confined phase of quarks and gluons • Experimental approach at RHIC • √sNN = 200GeV Au+Au collisions • Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions. • Penetrate the strong interacting matter • Emitted from every stage of collisions • Hard photons (High pT) • Initial hard scattering, Pre-equilibrium • Thermal photons (Low pT) • Carry the thermodynamic information from QGP and hadron gas thermal: Decay photons(p0→g+g, h→g+g, …) WWND 2008 @ South Padre Island

  3. Photon Measurement in PHENIX • Hard photon • Strong suppression of high pT hadrons helps to improve the S/N ratio • Successfully measured for both p+p and Au+Au collisions • Measured pT range • up to 14GeV/c • Good agreement with pQCD • Thermal photon • Thermal radiation from QGP • Primary contributor in low pT range up to 3-6GeV/c • Direct evidence of thermal equilibration • Thermal photon measurement is very challenging because it is very hard due to a large background from hadron decays. Au+Au S.Turbide et al PRC 69 014903 WWND 2008 @ South Padre Island

  4. p+p direct photon Low pT Photons e+ • Long-awaited results for both p+p and Au+Au • Experimental determination is very important since applicability of pQCD is doubtable in low pT region. • In ‘real’ photon measurement • Measured yield with a large systematic error • Difficulty on measuring low pT “real” direct photons • Finite energy resolution of the EMCal • Large hadron background q e- g* g q • Alternative method to measure low pT direct photons • Measure e+e- pairs from ‘virtual’ direct photons • Advantages on measuring ‘virtual’ photons • High momentum resolution of the Drift Chamber • Reliable estimation of the hadron decay components using Kroll-Wada formula WWND 2008 @ South Padre Island

  5. Virtual Photon Measurement • Any source of real g can emit g* with very low mass. • Convert direct g* fraction to real direct photon yield Kroll-Wada formula S : Process dependent factor • Case of Hadrons • Obviously S = 0 at Mee > Mhadron • Case of g* • If pT2>>Mee2 • Possible to separate hadron decay components from real signal in the proper mass window. WWND 2008 @ South Padre Island

  6. Signal Extraction p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • Real signal • di-electron continuum • Background sources • Combinatorial background • Material conversion pairs • Additional correlated background • Visible in p+p collisions • Cross pairs from decays with 4 electrons in the final state • Pairs in same jet or back-to-back jets WWND 2008 @ South Padre Island

  7. Hadronic Cocktail Calculation • Remaining pairs after background subtraction • Real signal + Hadron decay components • Estimate hadron components using hadronic cocktail arXiv: 0802.0050 • Mass distributions from hadron decays are simulated by Monte Carlo. • p0, h, h’, w, f, r, J/y, y’ • Effects on real data are implemented. • PHENIX acceptance, detector effect, efficiencies … • Hadronic cocktail was well tuned to individually measured yields of mesons in PHENIX for both p+p and Au+Au collisions. WWND 2008 @ South Padre Island

  8. Cocktail Comparison p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • p+p • Excellent agreement with cocktail • Au+Au • Large enhancement in low mass region • Integrated yield in150MeV < mee < 750MeV • Real/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model) Rencontres de Moriond - QCD and High Energy Interactions -

  9. 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c pT Sliced Mass Spectra Normalized by the yield in mee < 100MeV • Au+Au • p+p PHENIX Preliminary • Shape differences between p+p and Au+Au are larger at lower pT. WWND 2008 @ South Padre Island

  10. pT Dependence p+p Au+Au • Au+Au • pT<1GeV/c • Large enhancement → thermal qqbar and pp annihilations? • Au+Au • pT<1GeV/c • Large enhancement → thermal qqbar and pp annihilations? • pT>1GeV/c && mee<300MeV • Enhancement is also seen. • Small expected contributions from qqbar and pp annihilations • Satisfy pT2>>mee2 → Virtual photon analysis can be performed! WWND 2008 @ South Padre Island

  11. PHENIX Preliminary p+p Au+Au (MB) Cocktail Comparison 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV • p+p • Good agreement between real and cocktail • Small excess at higher pT • Au+Au • Good agreement in Mee < 50MeV • Enhancement is clearly seen above 100MeV. WWND 2008 @ South Padre Island

  12. Determination of g* fraction, r Direct g*/inclusive g* is determined by fitting the following function for each pT bin. Reminder : fdirect is given by Kroll-Wada formula with S = 1. r : direct g*/inclusive g* • Fit in 80-300MeV gives • Assuming direct g* shape • c2/NDF=11.6/10 • Assuming h shape instead of direct g* shape • c2/NDF=21.1/10 • Twice as much as measured h yield • Assumption of direct g* is favorable. Mee (GeV/c2) WWND 2008 @ South Padre Island

  13. direct g*/inclusive g* p+p Au+Au Base line Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang. μ = 0.5pT μ = 1.0pT μ = 2.0pT • p+p • Consistent with NLO pQCD • better agreement with small µ • Au+Au • Clear enhancement above NLO pQCD WWND 2008 @ South Padre Island

  14. Direct Photon Spectra The virtual direct photon fraction is converted to the direct photon yield. • p+p • First measurement in 1-4GeV/c • Consistent with NLO pQCD • Serves as a crucial reference • Au+Au • Above binary scaled NLO pQCD • Excess comes from thermal photons? WWND 2008 @ South Padre Island

  15. Theory Comparison D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) S.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004) T0ave=360 MeV (T0max=590 MeV) t0=0.15 fm/c T0max=370 MeV t0=0.33 fm/c WWND 2008 @ South Padre Island

  16. Summary & Outlook • Direct photon measurements with virtual photon method in p+p and Au+Au collisions have been done at RHIC-PHENIX. • The fractions of direct g* to inclusive g* above pT of 1GeV/c are obtained by making a shape comparison between real pairs and a hadronic cocktail. • This is the first time that direct photon production in p+p collisions has been measured in 1<pT<4GeV/c. • Direct photon yield in p+p collisions is consistent with NLO pQCD. • The result in p+p serves as a crucial reference to Au+Au result. • Excess of direct photon yield above binary scaled NLO pQCD in Au+Au collisions is observed. • The paper on direct photon measurement with virtual photon method will be submitted soon. • p+p analysis with more statistics is now ongoing. • Result with higher quality will be provided. • pT region will be extended upward. • Same analysis will be done in d+Au collisions. WWND 2008 @ South Padre Island

  17. backup WWND 2008 @ South Padre Island

  18. PHENIX Detector • Minimum Bias data sample (triggered by BBC) & Electron triggered data sample (p+p) • BBC • z-vertex • DC, PC1 • Tracking • RICH & EMCal • Electron ID • Electron Trigger WWND 2008 @ South Padre Island

  19. Conversion pair z B y MVD support structure r ~ mass e- e+ x f PHENIX Beam Pipe Material Conversion Pair Cut The pairs from material conversion should be removed. Dalitz decay These pairs can be recognized by its orientation relative to the magnetic field. z e- B y e+ x No cut M<30 MeV & fV<0.25 & M<600 MeV & fV<0.04 M<600 MeV & fV<0.06 M<600 MeV & fV<0.08 M<600 MeV & fV<0.10 M<600 MeV & fV<0.12 M<600 MeV & fV<0.14 M<600 MeV & fV<0.20 M<600 MeV & fV<0.40 WWND 2008 @ South Padre Island

  20. Additional Correlated Background Jet cross pair γ e- e+ e+ π0 e+ e- π0 π0 γ e- Dalitz + conversion cross pair γ e+ e- π0 γ e+ e- Correlated + combinatorial background is very good agreement with the real like sign mass spectrum. Systematic error due to background subtraction ≈ 2% WWND 2008 @ South Padre Island

  21. Combinatorial Background (Au+Au) • Normalization factor is determined by like sign pairs. • N++ and N-- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV) Normalization: 2√N++ N-- • Uncertainty due to statistics of N++ and N--: 0.12% WWND 2008 @ South Padre Island

  22. Centrality Dependence arXiv: 0706.3034 ● Au+Au ● p+p ▒ Cocktail • Integrated yield divided by Npart/2 • 150MeV<mee<750MeV • Strong centrality dependence • Increases faster than Npart • mee<100MeV (p0 region) • Agreement with cocktail WWND 2008 @ South Padre Island

  23. Theory Comparison 2 • Freeze-out Cocktail + “random” charm + r spectral function • Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: not reproduced WWND 2008 @ South Padre Island

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