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Di-electron Continuum at PHENIX

Di-electron Continuum at PHENIX. Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo. Introduction. Quark Gluon Plasma De-confined phase of quarks and gluons Predicted by lattice QCD Temperature higher than 150-200MeV Energy density higher than 1GeV/fm 3

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Di-electron Continuum at PHENIX

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  1. Di-electron Continuum at PHENIX Yorito Yamaguchi for the PHENIX collaboration CNS, University of Tokyo Rencontres de Moriond - QCD and High Energy Interactions -

  2. Introduction • Quark Gluon Plasma • De-confined phase of quarks and gluons • Predicted by lattice QCD • Temperature • higher than 150-200MeV • Energy density • higher than 1GeV/fm3 • Relevance to the history of the early universe • Properties of QGP are of keen interest • Strongly interacting system • Perfect fluid • Experimental approach • Heavy ion collisions at RHIC • √sNN = 200GeV Au+Au collisions Rencontres de Moriond - QCD and High Energy Interactions -

  3. hard: Photons & Di-leptons Compton e+ R. Rapp nucl-th/0204003 • Photons and di-leptons are emitted from thermally equilibrated medium. • Penetrating the strongly interacting matter • Carrying the thermodynamic information directly • Topics on di-electron continuum in low mass region (Below 1GeV) • Measurement of electron-pair yield in low-mass low-pT region in p+p and Au+Au • Enhancement possibly due to thermal q-qbar and p-p annihilations • Measurement of direct photon yield in p+p and Au+Au with virtual photon method • Thermal photons are primary contributor in 1-5GeV/c. q e- g* g q thermal: Decay photons(p0→g+g, h→g+g, …) Rencontres de Moriond - QCD and High Energy Interactions -

  4. Signal Extraction Au+Au 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 jet p+p After subtraction arXiv: 0802.0050 • Remaining pairs • Real signal + Hadron decay components • Estimate hadron components using hadronic cocktail • Tuned to individually measured yields of mesons at PHENIX Rencontres de Moriond - QCD and High Energy Interactions -

  5. Cocktail Comparison p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • p+p • Excellent agreement with cocktail • Au+Au • Large enhancement in 150MeV < mee < 750MeV • Integrated yield • Real/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model) Rencontres de Moriond - QCD and High Energy Interactions -

  6. 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 Rencontres de Moriond - QCD and High Energy Interactions -

  7. 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c Shape Difference • Au+Au • p+p PHENIX Preliminary • Shape differences between p+p and Au+Au are larger at lower pT. Rencontres de Moriond - QCD and High Energy Interactions -

  8. pT Dependence p+p Au+Au • p+p • Consistent with cocktail for all mass bins • Au+Au • Large enhancement at low pT as presented • p+p • Consistent with cocktail for all mass bins • Au+Au • Large enhancement at low pT • Enhancement is also observed in pT above 1GeV/c in the low-mass below 300MeV → Second topic of this presentation Rencontres de Moriond - QCD and High Energy Interactions -

  9. 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 Invariant mass of g* • Hadron mass distribution • Obviously S = 0 at Mee > Mhadron • g* mass distribution • If pT2>>Mee2 • Possible to separate hadron decay components from real signal in the proper mass window. Rencontres de Moriond - QCD and High Energy Interactions -

  10. 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. Rencontres de Moriond - QCD and High Energy Interactions -

  11. 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* mass shape • c2/NDF=11.6/10 • Assuming h shape instead of direct g* shape • c2/NDF=21.1/10 • Assumption of direct g* is favorable. Mee (GeV/c2) Rencontres de Moriond - QCD and High Energy Interactions -

  12. direct g*/inclusive g* p+p Au+Au 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 Rencontres de Moriond - QCD and High Energy Interactions -

  13. Direct Photon Spectra The virtual direct photon fraction is converted to the direct photon yield. • p+p • First measurement in 1-3GeV/c • Consistent with NLO pQCD • Serves as a crucial reference • Au+Au • Above binary scaled NLO pQCD • Excess from thermal photons? Rencontres de Moriond - QCD and High Energy Interactions -

  14. Summary & Outlook • Di-electron analyses in p+p and Au+Au collisions have been done at RHIC-PHENIX. • Large enhancement at low mass is observed in Au+Au collisions while the result in p+p collisions is consistent with a hadronic cocktail. • Strong centrality dependence • Large enhancement at low pT • 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. • Direct photon yield in p+p collisions is consistent with NLO pQCD. • First measurement of direct photons for 1<pT<3GeV/c in p+p collisions. • 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. • pT region of direct photon yield in p+p collisions will be extended upward with more statistics. • Same analysis will be done in d+Au collisions. Rencontres de Moriond - QCD and High Energy Interactions -

  15. backup Rencontres de Moriond - QCD and High Energy Interactions -

  16. PHENIX Detector • Minimum Bias data sample (triggered by BBC) & ERT triggered data sample • Analyzed events = 1.6B events • BBC • z-vertex • DC, PC1 • momentum • RICH & EMCal • Electron ID • Energy • Track Matching • Electron Trigger Rencontres de Moriond - QCD and High Energy Interactions -

  17. 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 Rencontres de Moriond - QCD and High Energy Interactions -

  18. 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% Rencontres de Moriond - QCD and High Energy Interactions -

  19. 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% Rencontres de Moriond - QCD and High Energy Interactions -

  20. Hadronic Cocktail Calculation (p+p) • 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 … arXiv: 0802.0050 • Parameterized PHENIX p0 data with assumption of p0 = (p++p-)/2 • Other mesons are measured individually. • Fit with mT scaling of p0 parameterization • Hadronic cocktail is well tuned to PHENIX data. Rencontres de Moriond - QCD and High Energy Interactions -

  21. Theory Comparison 1 • Freeze-out Cocktail + “random” charm + r spectral function • Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: not reproduced Rencontres de Moriond - QCD and High Energy Interactions -

  22. Theory Comparison 2 D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) T0ave=360 MeV (T0max=590 MeV) t0=0.15 fm/c Rencontres de Moriond - QCD and High Energy Interactions -

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