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Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration

Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration. Open Charm Production in s NN = 200 GeV Au+Au Collisions at STAR. 1. Outline. Motivations. Why we need to measure charm in heavy ion collisions. Open charm measured and published in d+Au collisions at STAR.

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Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration

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  1. Haibin ZhangBrookhaven National Laboratoryfor the STAR Collaboration Open Charm Production in sNN = 200 GeV Au+Au Collisions at STAR 1

  2. Outline Motivations Why we need to measure charm in heavy ion collisions Open charm measured and published in d+Au collisions at STAR Data Analysis D0 measurements in Au+Au collisions at sNN = 200 GeV TOF single electrons in Au+Au collisions at sNN = 200 GeV Results Charm cross-sections Charm nuclear modification factor J/ production in Au+Au Summary 2

  3. Motivation Charm quark mostly produced from the initial fusion of partons (mostly gluons)  Open charm production sensitive to initial parton distribution functions 2001, Dokshitzer and Kharzeev proposed “dead cone” effect => charm quark small energy loss, but in vacuum Recent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027,2005;M. Djordjevic et al., PRL 94, 112301, 2005. Z. Lin & M. Gyulassy, PRC 51 (1995) 2177 light 3

  4. Open Charm Measured in d+Au Phys. Rev. Lett. 94 (2005) 062301 Scaled by Nbin to p+p NLO pQCD calculations tuned for low energy points under-predict the ccbar production cross section at RHIC 4

  5. D0 Measurement Technique Data Set ~14M minimum bias Au+Au events at sNN = 200 GeV 0-80% collision centrality selected Event mixing technique Select K and  tracks from PID by energy loss in TPC Combine all pairs from same event  Signal+Background • Combine pairs from different events Background • Signal = same event spectra – mixed event spectra • More details about this technique can be found at • PRC 71 (2005) 064902 and PRL 94 (2005) 062301 5

  6. D0 Signal Event-mixing background subtracted invariant mass spectrum fit with gaussian + Linear function A 4.5 D0 signal observed at pT<3 GeV and |y|<1 from 14M Au+Au minbias collision events Three signals also seen in sub-pT ranges with ~2.5 significances 6

  7. TOF electron measurements 2/ndf = 65/46 2/ndf = 67/70 Electrons can be separated from pions. But the dEdx resolution is worse than d+Au Log10(dEdx/dEdxBichsel) distribution is Gaussian. 0.3<pT<4.0 GeV/c |1/-1|<0.03 2 Gauss can not describe the shoulder shape well. • Exponential + Gaussian fit is used at lower pT region. • 3 Gaussian fit is used at higher pT region. 7

  8. Dominant source at low pT Mass(e+e-)<0.15 GeV/c2 Electron Spectrum γ conversion π0Dalitz decay η Dalitz decay Kaon decay vector meson decays • Combinatorial background reconstructed by track rotating technique. • Invariant mass < 0.15 for photonic background. 8

  9. Charm pT Spectra D0 and e combined fit Power-law function with parameters dN/dy, <pT> and n to describe the D0 spectrum Generate D0e decay kinematics according to the above parameters Vary (dN/dy, <pT>, n) to get the min. 2 by comparing power-law to D0 data and the decayed e shape to e data <pT>=1.20  0.05(stat.) GeV/c in minbias Au+Au <pT>=1.32  0.08(stat.) GeV/c in d+Au 9

  10. Charm Total Cross Section Charm total cross section per NN interaction 1.13  0.09(stat.)  0.42(sys.) mb in 200GeV minbias Au+Au collsions 1.4  0.2(stat.)  0.4(sys.) mb in 200GeV minbias d+Au collisions Charm total cross section follows roughly Nbin scaling from d+Au to Au+Au considering errors Indication of charm production in initial collisions 10

  11. Single Electron Spectra Single electron spectra also measured in three centralities in Au+Au collisions Single electron spectra in Au+Au compared with the D0e curve in d+Au scaled by Nbin The single electron data points in central and minbias Au+Au lower than the D0e curve in d+Au scaled by Nbin 11

  12. Charm Nuclear Modification Factor RAA suppression for single electron in central Au+Au similar to charged hadrons at 1.5<pT<3.5 GeV/c Charm transverse momentum spectra modified by the hot and dense medium in central Au+Au collisions at RHIC Suppressions!! STAR: Phys. Rev. Lett. 91 (2003) 172302 12

  13. 12M events STAR Preliminary 12M events STAR Preliminary J/ production Select all possible electrons, provided they don’t impinge any of the hadron bands The di-electron invariant mass distribution is generated using the event-mixing The J/ yield is lower than statistical coalescence model prediction (red-dashed curve, A. Andronic)  Extreme enhancement scenario ruled out 13

  14. J/ yield significantly lower than statistical coalescence prediction based on the above charm total cross-section Nuclear modification factor of single electrons is suppressed in central Au+Au similar to charged hadrons at 1.5<pT<3.5 GeV/c Heavy flavor production IS modified by the hot and dense medium in Au+Au collisions at RHIC Charm total cross section in Au+Au 1.13  0.09  0.42 mb consistent with d+Au 1.4  0.2  0.4 mb within errors  Nbin scaling for total cross section D0 signals observed in minbias Au+Au collisions at 200 GeV The inclusive and photonic electron spectra presented. Significant excess above the background in Au+Au 200 GeV. Expected to becontribution of semi-leptonic decay from heavy flavor Summary 14

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