Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

Haibin ZhangBrookhaven National Laboratory

for the STAR Collaboration

Open Charm Production in Open Charm Production in ssNN NN = 200 = 200 GeV Au+Au Collisions at STARGeV Au+Au Collisions at STAR

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Haibin Zhang Quark Matter 2005, August 8th, 2005

STAROutline

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Motivations

Open charm measured and published in d+Au collisions at STAR

D0 measurements in Au+Au collisions at ssNNNN = 200 GeV = 200 GeV

TOF single electrons in Au+Au collisions at ssNNNN = 200 GeV = 200 GeV

Summary

Why we need to measure charm in heavy ion collisions

Charm cross-sectionsCharm nuclear modification factorJ/ production in Au+Au

Data Analysis

Results

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

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MotivationCharm quark mostly produced from the initial fusion of partons (mostly gluons) Open charm production sensitive to initial parton distribution functions

Z. Lin & M. Gyulassy, PRC 51 (1995) 2177

light

2001, Dokshitzer and Kharzeev proposed “dead cone” effect => charm quark small energy loss, but in vacuumRecent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027, 2005; M. Djordjevic et al., PRL 94, 112301, 2005.

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

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Open Charm Measured in d+Au

NLO pQCD calculations tuned for low energy points under-predict the ccbar production cross section at RHIC

Phys. Rev. Lett. 94 (2005) 062301

Scaled by Nbin to p+p

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

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D0 Measurement Technique

Event mixing techniqueSelect K and tracks from PID by energy loss in TPCCombine all pairs from same event Signal+Background

Combine pairs from different events BackgroundSignal = same event spectra – mixed event spectra

Data Set~14M minimum bias Au+Au events at ssNNNN = 200 GeV = 200 GeV0-80% collision centrality selected

More details about this technique can be found atPRC 71 (2005) 064902 and PRL 94 (2005) 062301

Haibin Zhang Quark Matter 2005, August 8th, 2005

STARD0 Signal

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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 rangeswith ~2.5 significances

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

~7.6M AuAu 200GeV Run IV P05ia production 0~80% Min. Bias. |Vz| < 30cm

Electrons can be separated from pions. But the dEdx resolution is worse than d+Au

Log10(dEdx/dEdxBichsel) distribution is Gaussian.

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.

2/ndf = 65/46

0.3<pT<4.0 GeV/c

TOF electron measurements

|1/-1|<0.03

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2/ndf = 67/70

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR

Mass(e+e-)<0.15 GeV/c2

Combinatorial background reconstructed by track rotating technique.

Invariant mass < 0.15 for photonic background.

γ conversion π0 Dalitz decay η Dalitz decay Kaon decay vector meson decays

Dominant source at low pT

Electron Spectrum

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Haibin Zhang Quark Matter 2005, August 8th, 2005

STARCharm pT Spectra

Power-law function with parameters dN/dy, <pT> and n to describe the D0 spectrum

D0 and e combined fit

Generate D0e 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

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Haibin Zhang Quark Matter 2005, August 8th, 2005

STARCharm Total Cross Section

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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 per NN interaction

Charm total cross section follows roughly Nbin scaling from d+Au to Au+Au considering errors

Indication of charm production in initial collisions

Haibin Zhang Quark Matter 2005, August 8th, 2005

STARSingle Electron Spectra

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Single electron spectra also measured in three centralities in Au+Au collisions

Single electron spectra in Au+Au compared with the D0e curvein d+Au scaled by Nbin

The single electron data points in central and minbias Au+Au lower thanthe D0e curve in d+Au scaled by Nbin

Haibin Zhang Quark Matter 2005, August 8th, 2005

STARCharm Nuclear Modification Factor

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STAR: Phys. Rev. Lett. 91 (2003) 172302

Suppressions!!

RAA suppression for single electron incentral Au+Au similar to charged hadrons at 1.5<pT<3.5 GeV/c

Charm transverse momentum spectramodified by the hot and dense mediumin central Au+Au collisions at RHIC

Haibin Zhang Quark Matter 2005, August 8th, 2005

STARJ/ production

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Select all possible electrons, provided they don’t impinge any of the hadron bandsThe di-electron invariant mass distribution is generated using the event-mixing

12M eventsSTAR Preliminary

The J/ yield is lower than statistical coalescence model prediction (red-dashed curve, A. Andronic) Extreme enhancement scenario ruled out

12M eventsSTAR Preliminary

Haibin Zhang Quark Matter 2005, August 8th, 2005

STARSummary

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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 be contribution of semi-leptonic decay from heavy flavor

Charm total cross section in Au+Au 1.13 0.09 0.42 mb consistent withd+Au 1.4 0.2 0.4 mb within errors Nbin scaling for 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

J/ yield significantly lower than statistical coalescence prediction based on the above charm total cross-section

Heavy flavor production IS modified by the hot and dense medium in Au+Au collisions at RHIC

Haibin Zhang Quark Matter 2005, August 8th, 2005

STAR