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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

Haibin ZhangBrookhaven National Laboratoryfor the STAR Collaboration

Open Charm Production in sNN = 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

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


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


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


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


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


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


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


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


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


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


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


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


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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