# Presentation: 942450

## Heavy Flavor Measurements at STAR

- Haibin Zhang
- Brookhaven National Laboratory
*for the STAR Collaboration*

## Motivation – Charm Production Mechanism

-  Our final goal is to understand the properties of the hot and dense matter produced in heavy ion collisions

-  Charm can provide a unique tool to study important properties of the new matter

-  However, we have to understand the charm production mechanism first: initial parton fusion, flavor excitation,  etc.

-  Theorists believe charm is mainly produced in initial collisions via gluon fusion in relativistic heavy ion collisions (M. Gyulassy & Z. Lin, PRC 51 (1995) 2177)  charm total cross-section should follow Nbin scaling from p+p to Au+Au

-  It’s important to measure charm total cross-section in Au+Au and compare to that in p+p and d+Au

## Motivation – Charm vs. Thermalization

-  Charm (Moore and Teney, PRC 71(2005) 064904) or “charm resonance” (Hees and Rapp, PRC 71(2005) 034907) interact with the medium via scattering:

-  Its phase space shape may be changed at low pT (<3-5 GeV/c)

-  Charm could pick up elliptic flow from the medium

-  Measurements of charm pT spectra and elliptic flow may give us hint that the partonic matter could be thermalized

## Motivation – Charm Energy Loss

-  In 2001, Dokshitzer and Kharzeev proposed “dead cone” effect  charm quark small energy loss

-  Recent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027, 2005; M. Djordjevic et al., PRL 94, 112301, 2005.

-  Heavy quarks will be important to understand the Energy Loss mechanisms and the competition between them

-  Mechanisms other than gluon emission may play an important role for heavy quark energy loss

##  Hadronic decay channels: D0K(B.R.: 3.8%)

 Semileptonic channels:
 c  l+ + anything (B.R.: 9.6%)
D0  e+ + anything	(B.R.: 6.87%) 
D0  + + anything	(B.R.: 6.5%)

-  Semileptonic channels:
-  **c ******** ****l****+**** + anything (B.R.: 9.6%)**
**D****0 ******** e****+**** + anything****	****(B.R.: 6.87%)**** **
**D****0**** ******** ********+**** + anything****	****(B.R.: 6.5%)**

- What STAR Measures

## Detailed Measurements

-  D0K in d+Au and Au+Au (0<pT<3 GeV/c, |y|<0.5)

-  Heavy-flavor decayed single electron spectra in p+p, d+Au and Au+Au

-  Charm decayed muon spectra in Au+Au (0.17<pT<0.25 GeV/c, -1<<0)

-  H. Zhang, QM2005

-  STAR paper, PRL 94 (2005) 062301

-  Measured by Time-of-Flight Detector (1<pT<4 GeV/c, -1<<0)

-  Measured by Barrel EMC Detector (1.2<pT<10 GeV/c, 0<<1)

-  C. Zhong, this conference

-  H. Zhang, QM2005

-  STAR paper, PRL 94 (2005) 062301

-  J. Bielcik, QM2005 and this conference

## Combined Fit

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

- D0, e ,  combined fit

- 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
- and  data

- Advantage: D0 and  constrain low pT
- e constrains higher pT

- Spectra difference between e and  ~5% (included into sys. error)

## Charm Total Cross Section

- 1.26  0.09  0.23 mb
- in 200 GeV minbias Au+Au

- 1.4  0.2(stat.)  0.4(sys.) mb
- in 200 GeV minbias d+Au

- Charm total cross section
- per NN interaction

- Charm total cross section follows Nbin scaling from d+Au to minbias Au+Au to central Au+Au considering errors

- Supports conjecture that charm is exclusively produced in initial scattering

- 1.33  0.06  0.18 mb
- in 200 GeV 0-12% Au+Au

- However, the total cross section is a factor of ~5 larger than NLO predictions!!!

- STAR Preliminary

## Discrepancy at RHIC?

*PHENIX, PRL 94, 082301(2005)*

## Blast-Wave Fit – Charm Freeze-Out

- Blast-wave fit combining D0, muons, and electrons at pT<2 GeV/c

- Charm hadrons may freeze-out earlier – T>140 MeV

- Charm hadron collective velocity <T> less than that of  and  - charm flow?

**STAR Preliminary**

**STAR Preliminary**

## RAA of Non-Photonic Electrons

*STAR Highlight talk*

- PHENIX, PRL96

- RAA of Non-Photonic Electrons

- J. Bielcik, this conference

## Nuclear Modification Factor - EMC

-  However, the amount of beauty contributions to electrons is still uncertain!!

-  We need to measure RAA from Ds directly     to clarify

-  Theories currently do not describe the data

-  Only charm contribution would describe the RAA but not the p+p spectra

- J. Bielcik, this conference

## Detector Upgrate – Heavy Flavor Tracker

- A silicon detector, can provide a ~50m DCA
- resolution to reconstruct secondary decay
- vertices of charm hadrons

- Simulation with 1.43M central Au+Au events

## Summary

-  Charm total cross section per NN collision follows Nbin scaling from d+Au to      minbias Au+Au to central Au+Au  charm produced via initial parton fusion

-  Strong suppression of non-photonic electron RAA at high pT observed in central Au+Au collisions  Challenge to existing energy loss models

-  Charm transverse momentum distribution has been modified by the hot and dense medium in central Au+Au collisions!!!

-  Blast-wave fit to charm spectra  small <>, large Tfo  charm hadrons may freeze-out earlier