# Presentation: 771795

## Charm cross-sections and collectivity from its semileptonic decay at RHIC

- Charm cross-sections and collectivity from its semileptonic decay at RHIC

- Yifei Zhang
- University of Science & Technology of China
- Lawrence Berkeley National Laboratory

| Outline
Charm cross-sections
        STAR measurements
          low pT muon
          binary scaling
Charm collectivity
          decay form factor
          lepton spectra
          bottom contributions
Summary |
| --- |

- H.D. Liu, et al., nucl-ex/0601030
- Many thanks to:
- X. Dong, S. Esumi, Z. Xu, H. Zhang, C. Zhong, N. Xu

## At low pT ...

- At low pT ...

## Why measure charm cross-section?

- Mc  1.5 GeV >> Tc, QCD, Muds                   M/E >> 0

- Charm quarks are believed to be produced at early stage by initial gluon fusions. (M. Gyulassy & Z. Lin, PRC 51 (1995) 2177)

- Charm total cross-section should follow number of binary collisions (Nbin) scaling.

## Open heavy flavor measurements at RHIC

- PHENIX non-photonic electron – integral cross-section at 0.8<pT<4 GeV/c.
- PHENIX forward muon – also pT>1 GeV/c. (Xiaorong Wang, SQM06)
- STAR direct D-meson reconstruction – large combinatorial b.g.
- STAR EMC non-photonic electron – pT>2 GeV/c.
- STAR TOF non-photonic electron – large conversion b.g. at low pT.
- STAR low pT muon – pT~0.2 GeV. Good constraint to charm cross-section.

- STAR PRL 94, 062301 (2005)

- X. Dong USTC

## We need accurate measurement

- Systematic errors dominant!

*PHENIX, PRL 94, 082301(2005)*

**An accurate measurement of charm total cross-section ******
**test pQCD, N****bin**** scaling, J/******** production, etc...**

##    How can muon constrain charm x-section?

- The muons in 0.16<pT<0.26 GeV/c sample 14% of the charmed hadron spectrum besides 10.5% b.r..
- The similar spectral shape shows that the muon uniformly samples the entire charmed hadron spectrum.

- The electron spectrum at pT>1 GeV/c can not constrain charm yields.
- The low pT muon yields are uniform from whatever shape of the charmed hadron.

## Low pT muon method

**muon candidates** and **primary pion candidates **after TPC dE/dx selections.

**Primary particle DCA** and **muon DCA from pion/Kaon decay** after TPC dE/dx and TOF m2 selections

- from HIJING simulation through realistic STAR detector configuration.

##   Centrality dependence of charm cross-section

- Haibin Zhang/Yifei Zhang, SQM06

- Low pT  constrains charm cross section.
**Within error bars, consistent with binary scaling. **
**Charm produced at the initial impact.**
- Larger than PHENIX data and pQCD.

- Charm cross-section at mid-rapidity were extracted by a combined fit to D0, muon and electron spectra.

## Charm collectivity

-  decay form factor
-  lepton spectra
-  bottom contributions

- At higher pT ...

## Charm leptonic decay form factors

- PYTHIA uses a simplified vector meson decay form factor – softer
- PDG form factor is from three decay channels:

- Leptonic decay form factor is much harder than PYTHIA. Up to a factor of 1.5 around pT 3-4 GeV/c.
- Consistent with other decay form factors.
*M. Cacciari, P. Nason and R. Vogt, **PRL 95 (2005) 122001*
*CLEO Collaboration, J. Yelton Heavy Quarks and Leptons June 4,2004*

## Are lepton spectra sensitive to flow?

*S. Batsouli, et al, Phys.Lett.B 557 (2003) 26*

-  Electron spectra cannot disentangle different charm hadron shapes at low pT

## Charm collectivity from BW

- BlastWave model-dependent fit shows charm has collectivity.
- The sensitivity of electron spectrum to charm spectrum shape is weak.

- Haibin Zhang/Yifei Zhang, SQM06

## Assumption for bottom contributions

- NLO, FONLL prediction:
- R. Vogt, hep-ph/0502203
- bbar / ccbar = 0.2% ~ 2.6%

- Since non-photonic electron strongly lose energy, bottom contribution seems small in Au + Au collisions.
- So we assume bbar / ccbar = 0.3%~0.7%

## Lepton v2 from heavy flavor hadron decay

- Lepton v2 from heavy flavor hadron decay

- (a) X. Dong, *et al*., PLB 597 (2004) 328
- (b) Assumed c/b-hadron v2
- (c) Decayed electron v2.
- (d) c+b  e v2 compare to PHENIX data

| B-v2 | c+b  e v2 | Consistent with RAA? | Consistent with PHENIX v2? |
| --- | --- | --- | --- |
| v2b = v2q | Red diamond | NO | NO |
| v2b = 0 | Black open square | NO | YES |

## Bottom contribution is negligible!!

- The decayed electron v2 from assumed decreasing c-hadron v2 can describe PHENIX data. **YES**
- Consistent with energy loss!  **YES**
**Bottom contribution is negligible **
**up to p****T****~5 GeV/c.**

## Summary

** ****We develop the low p****T**** (~2 GeV/c) muon method to constrain charm cross-sections. **Charm cross-sections at mid-rapidity are extracted from a combination of the three different measurements at STAR.
-     The cross-section is found to follow binary scaling within error bars. It is larger than PHENIX data and pQCD calculation.
** **The charm decay form factor was studied.** **BlastWave model-dependent fit shows charm has collectivity.
-     Comparison between simulation and PHENIX data shows charm flow strongly.
-     From our assumptions, PHENIX non-photonic electron v2 can be reproduced by simulation without bottom contribution. **Bottom contribution is negligible up to p****T****~5 GeV/c.**

## Backup slides

## PHENIX charm cross-section

*PHENIX, PRL 94, 082301(2005)*

**Run2pp ********-**
**FONLL: Solid line and  band**
**PYTHIA w/o scaling the charm contribution: dotted line**

- PHENIX forward muon also more than 5 times of pQCD. (Xiaorong Wang, SQM06)

## RAA: how the electron sensitive?

## How high pt can BW fit to?

- Charm hadron: pT<~ 3 GeV/c electron: pT<~2 GeV/c
- Sensitive to different freeze-out: T=120MeV, m=0.7T=220MeV, m=0.36
- STAR & PHENIX data consistent