# Presentation: 715208

## System Size Dependence of Strange Hadron Elliptic Flow from 200 GeV Au + Au and Cu + Cu collisions 

*Yan Lu *
*for STAR collaboration**   *
**Department of Modern Physics **
**University of Science and Technology of China**

**Quark Matter 2008**
**The 20****th**** International Conference on **
**Ultra-relativistic Nucleus-Nucleus Collisions**
**February 4–10, Jaipur, India**

## Outline

-  Motivation
-  Data analysis
-  Results and Discussion
-  Summary and Outlook

## Thermalization?

- Data: S. Voloshin, nucl-ex/ 0701038
- Hydro: Kolb, Sollfrank, Heinz, PRC 62 (2000) 054909

-  Possible thermalization only for the most central collisions.

**    ‘****We argue that the centrality dependence of elliptic flow should be a good indicator of the degree of equilibration reached in the action. ****’**
*                                                                                  **S. Voloshin, A. Poskanzer, PLB 474, 27, 2000.*

## Partonic Collectivity at RHIC

** **
-       **‘****They are made via coalescence of seemingly thermalized quarks in central Au+Au collisions, the observations imply *****hot and dense matter with partonic collectivity***** has been formed at RHIC’**
*                                                   **STAR:  Phys. Rev. Lett., 99, 112301(07), nucl-ex/0703033*

*minimum bias*

## Why System Size and Centrality Dependence?

-                             *versus*
- - Systematic study for identified hadrons will gain information on interplay of collectivity, NQ scaling and thermalization as a function of collision system size and centrality.
-  Determine parameters for partonic EoS.

- System Size

- Centrality

## Dataset and Cuts

- multiplicity   collision centrality

**Event cuts: |Vertex z| < 30cm**
**FTPC track cuts: 6 =< nFitHits < 12, nHitsFit/nHitsPoss > 0.52**
**                                   ****p****T**** > 0.1, 2.5 < |********| < 4.0, Dca < 2.0 cm**

## PID and v2 Methods

- PID and v2 Methods

-  v2 vs. minv method (background subtraction technique):
-     - used for K0S, Λ, Ξ, , and  analysis and for Au+Au and Cu+Cu datasets
-  flow method
-      - Event Plane method used for Au+Au and Cu+Cu datasets
-      - non-flow systematic: Lee-Yang Zero method for Au+Au and Event Plane method with FTPC reaction plane for Cu+Cu

- Lee-Yang Zero method: R. S. Bhalerao, N. Borghini, J.-Y.Ollitrault, Nucl. Phy. A 727 (2003) 373-426
- Event Plane method: A. M. Poskanzer, S. A. Voloshin, Phys. Rev. C58, 1671 (1998)

## Centrality Dependence ofIdentified  v2 

- Centrality Dependence ofIdentified  v2

** ****All centralities: mass ordering at low p****T**** ******** ****m****T****-m scaling at low m****T****-m. **
** ****All centralities: baryon v****2 ****> meson v****2**** at intermediate p****T ****or m****T****-m.**

- STAR Preliminary

**Transverse momentum**

*Hydro: P. Hu**ovinen, private communications, 2007*

- 200 GeV Au+Au

## Centrality Dependence of Number-of-Quark Scaling 

- Centrality Dependence of Number-of-Quark Scaling

-  nq scaling is observed at all centrality bins.

- 200 GeV Au+Au

- STAR Preliminary

**n****q****: number of quark within a hadron**

## Eccentricity Scaling(?)

- STAR Preliminary

-  Not observe the part scaling claimed by PHENIX.
-  Divided v2 by <v2>ch instead of part, it appears that the scaling works better.
-  Scale by part to remove initial geometry effect.
-  Larger v2/part indicates stronger flow in more central collisions.

- PHENIX: Phys. Rev. Lett 98, 162301 (2007)
- Phys. Lett. B 503, 58 (2001)

- 200 GeV Au+Au

- 200 GeV Au+Au

## pT-integrated v2/part

- STAR Preliminary

- Charge particle data: STAR, Phys. Rev. C 72, 014904, 2005.

-  v2/part versus Npart
-         data: increasing trend indicates stronger flow in more central collisions.
-         hydro: little sensitivity to the collision centrality as expected in equilibrium scenario.
-  v2/part for a given centrality
-         hydro: a clear hadron mass dependence.
-         data: not clear due to large errors.
-  Above Npart ~ 170, integrated v2 consistent with hydro prediction indicates local thermalization.

- 200 GeV Au+Au

## System Size Dependence

- STAR Preliminary

** ****A given colliding system: clear scaling with n****q****.**
** ****System size dependence:**
**     ****- No ********part  ****scaling claimed by PHENIX**
**     ****- v****2**** seems to fall at lower p****T**** in Cu+Cu than in Au+Au**

- PHENIX: Phys. Rev. Lett 98, 162301 (2007)

**See S. S. Shi’s poster**

## Summary

-  We report azimuthal anisotropy parameter v2 of strange and multi-strange hadrons from Au+Au and Cu+Cu collisions at 200 GeV. Detailed centrality dependence is presented up to pT ~ 6 GeV/c.
-  At low pT region, collectivity is observed at all centrality bins under study.
-           it follows a mT-m scaling.
-  Not observe the part scaling.
-          larger v2/part indicates stronger flow in more central collisions and larger system.
-  v2/part increase with Npart, reaching hydro prediction in central Au+Au collisions.
-          indicate thermalization in central Au+Au collisions.

## Future:

- More systematic measurements for , : partonic EoS parameters
- Heavy quark collectivity: early thermalization of light quarks

## Backup slides!

## Elliptic flow

- impact parameter

- beam

## v2 vs minv method

- analysis method:   **v****2 ****vs. m****inv**
- motivated by
- 	Borghini et al. [nucl-th/0407041]
- advantages over standard method:
- only one fit per pT bin
- -   smaller systematic uncertainties
- method used for K0S, Λ, Ξ, , and  analysis
- standard method and v2 vs. minv method give consistent results

- SIG  +  BG

- BG

- BG
- (SIG+BG)

- SIG
- (SIG+BG)

*v**2**TOT*(*m**inv*)

## Systematic Errors Evaluation

-     - measure flow using genuine correlations among all particles.
-     - flow determined from first minimum r0 of generating function.

-  Standard method (Event Plane method):
-     - flow vector                      - event plane angle

- Sensitive to non-flow.

-  FTPC Reaction Plane:
-     - Similar to Event Plane method
-     - Correlate particles in TPC region with FTPC reaction plane.

- A. M. Poskanzer, S. A. Voloshin, Phys. Rev. C58, 1671 (1998)

- R. S. Bhalerao, N. Borghini, J.-Y.Ollitrault, Nucl. Phy. A 727 (2003) 373-426

- Suppress non-flow.

- all particles

- arbitrary variable

## Non-flow systematics

-  200 GeV Au+Au 10-40%.
-  h± (yellow bands): decreasing trend.
-  PID: no clear pT and PID dependence.
-  v2{LYZ} < v2{EP}: ~10% effect.

-  200 GeV Cu+Cu 0-60%.
-  v2{EP, FTPC} < v2{EP, TPC}.
-  larger effect at higher pT.
-  v2 from two methods consistent for  within large errors.

## System Size and Centrality Dependence

- STAR Preliminary

- PHENIX: Phys. Rev. Lett 98, 162301 (2007)

-  A given centrality of a colliding system: clear scaling with nq.
-  A given system: stronger flow in more central collisions.
-  Not observe the part scaling claimed by PHENIX, which is independent on colliding system .