# Presentation: 122332

## System Size Dependence of-meson Production at RHIC

- Xinghua Shi (LBNL/SINAP)
- STAR Collaboration

## Outline

**Motivation**
- ******-meson production**
**-**** ********-meson**** v****2**** from different system size at 200GeV**
**Summary**

**1**

## Motivation 

- Some features of ****-meson :
- Mass  : 1019.456 +/- 0.020 MeV
- Width : 4.26 +/- 0.05 MeV
- Decay Mode :  K+K-   49.2%
**C    ~** 45fm (longer than fireball lifetime)
- Quark Structure : s sbar (referred to as hidden strangeness)
**Small hadronic interaction cross sections**

****** ****production can be a good probe**

**Information from earlier time**
**Sensitive to partonic flow**

**2**

**We have measured the productions of ********meson for various colliding energies and systems at RHIC**

## Transverse Momentum Spectra and Ratios

**3**

**Bulk production of the ********-meson follow N****part**** scaling at RHIC**

**STAR  preliminary**

**STAR  preliminary**

## Strangeness Enhancements at RHIC  

**4**

- The enhancement seems scaled by the Npart
- Enhancement of -meson (*ss*) is in between that of  (*sqq*) and  (*ssq*) at 200 GeV
- The enhancement is larger at 200 GeV than that at 62.4 GeV heavy ion collisions
-   Inconsistent with canonical suppression model predictions.

**STAR  preliminary**

**STAR  preliminary**

## Anisotropy Parameter v2

- y

- x

- py

- px

- coordinate-space-anisotropy              momentum-space-anisotropy

- Anisotropy Parameter v2

**Initial/final conditions, EoS, degrees of freedom**

##  Reaction Plane Determination in STAR

**              **

** ****6**

**Why we use FTPC reaction-plane?**
**1.The non-flow correlation and auto correlation effects are smaller from FTPC reaction-plane**
**2.Comparing the v****2**** results from FTPC reaction-plane with TPC reaction-plane can help us understand the non-flow effects.**

**TPC**** or**** FTPC**
** ****reaction-plane**

## Shifting Corrections

- Even with the weighting method, the event plane distribution from FTPC we obtain isn't isotropic. We can flatten that distribution by shifting:

-     (n=1,2,3,4...... )

**7**

*   **J. Barrette et al. *Phys.Rev. **C56**, 3254(1997), *nucl-ex/9707002*

- Ψ(rad**)**

**counts**

## Reaction Plane from FTPC and TPC

**Reaction Plane from FTPC and TPC**

**Before weight corrections**
**Before shifting**
**After shifting**
**Fit with a line**

**Before weight corrections**
**Before shifting**
**After shifting**
**Fit with a line**

** ****8**

**(a) W-FTPC**

**(b) E-FTPC**

**Before shifting**
**After shifting**
**Fit with a line**

**Full-FTPC: cos(2△****Ψ****):0.335   **

**TPC: cos(2△****Ψ****):  0.604**

**(c) Full-FTPC**

**(d) TPC**

**(rad)**

**(rad)**

**(rad)**

**(rad)**

## Results: -meson v2 at 200GeV

**First observation of the elliptic flow vs. p****T**** in Cu+Cu collisions.**
******-****meson v****2**** might be negative value in Cu+Cu collisions. In high p****T**** non-flow effects make v****2**** greater.**
**The v****2**** from Au+Au collisions is greater than that from Cu+Cu at 200GeV**

**9**

**STAR Preliminary**** (15M)**

**STAR  preliminary (15M)**

**STAR  preliminary **

## Summary

-     **1) We reported the first observation of the ********-meson elliptic flow vs. p****T**** in Cu+Cu collisions.**
**    ****2) At low p****T ********-meson v****2**** in Cu+Cu collisions from TPC and FTPC reaction-plane**** ****are similar. At high p****T**** ****the differences are obvious because of non- flow effects.**
**    **
**     ****Next:  (i) Analyze systematic effects**
**                ****(ii) Including HT trigger data set.**

**10**

- Thanks!