1. Anisotropic Flow at RHIC Jiayun Chen (for Collaboration) Institute of Particle Physics, HZNU, Wuhan, 430079, P.R.China Brookhaven National Lab, Upton, NY,11973, USA

2. Jiayun Chen for STAR Collaboration – QNP2009 2 Outline Introduction and Motivation STAR Experiment Result and discussions Partonic Collectivity Ideal Hydrodynamic Indentified Particles’ (PID) Directed Flow Summary and outlook

3. Jiayun Chen for STAR Collaboration – QNP2009 3 This is believed to be due to pressure gradients in the dense medium which lead to collective motion. Directed flow (v 1 ) at forward rapidities describes the “side-splash”, the foremost early collective motion of the system; Elliptic flow (v 2 ) demonstrated the development of collectivity Provide insights into the EOS. Coordinate-Space Anisotropy Momentum-Space Anisotropy interactions pressure gradient  collective flow Azimuthal Anisotropy

4. Jiayun Chen for STAR Collaboration – QNP2009 4 STAR Detector 2.5<|η|<4.0 |η|<1.5 ZDC-SMD

5. Jiayun Chen for STAR Collaboration – QNP2009 5 Partonic Collectivity 1)At low p T - mass dependence 2)At intermediate p T - clear difference between baryons and mesons 3) Hadrons with u-, d-, s-quarks show similar collectivity Final word on partonic collectivity at RHIC! PHENIX π and p: nucl-ex/0604011v1 NQ inspired fit: X. Dong et al. Phy. Let. B 597 (2004) 328-332

6. Jiayun Chen for STAR Collaboration – QNP2009 6 STAR preliminary Ideal Hydro Test (1) Ideal hydro: P. Huovinen, private communication Ideal hydro fails to reproduce the data. Fluctuation of v 2 ? Viscosity ? Incomplete thermalization ? Au+Au at 200 GeV Phys. Rev. C 77 (2008) 54901

7. Jiayun Chen for STAR Collaboration – QNP2009 7 Ideal Hydro Test (2)  v 4 /v 2 2 results suggest that ideal hydro limit is not reached. Borghini & Ollitrault, PLB 642 227 (2006) Au+Au at 200 GeV 0-80%

8. Jiayun Chen for STAR Collaboration – QNP2009 8 Stopping and space-momentum correlation R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84 (2000), 2803. space-momentum correlations, nucleus stopping  wiggle structure of v 1 (y) develops

9. Jiayun Chen for STAR Collaboration – QNP2009 9 Anti-flow/3rd flow component flow antiflow Directed flow (v 1 ) and phase transition,QGP  v 1 (y) flat at mid-rapidity Phys. Rev. C 61 (2000), 024909.

10. Jiayun Chen for STAR Collaboration – QNP2009 10 PID Directed Flow v 1 First measurement of directed flow of antiprotons. Proton directed flow "collapses". Antiproton v 1 has the same sign of that of pions - the collapse of proton v 1 is not a mass effect. Kaon has a smaller k/p cross section than that of pions, thus it suffers less shadowing effect, yet we found negative v 1 slope for both charged kaon and kshort- consistent with "anti-flow" picture. Proton 0.4<p T <1.0 (GeV/c) Antiproton 0.4<p T <1.0 (GeV/c) Pion 0.15<p T <0.75 (GeV/c) Kaon 0.2<p T <0.6 (GeV/c) STAR preliminary

11. Jiayun Chen for STAR Collaboration – QNP2009 11 Directed Flow v 1 Proton v 1 slope is close to zero, if compared to others. The models with shadowing effects (RQMD) cannot explain the v 1 slope relative to that of pions. Phys. Rev. Lett. 84 (2000) 2803; Phys. Lett. B 526 (2002) 309–314; Phys. Rev. C 71, 054905 (2005). STAR preliminary

12. Jiayun Chen for STAR Collaboration – QNP2009 12 Charged hadrons Directed Flow v 1 At mid-rapidity, all the results have comparable values. At forward rapidity, the trend of v 1 from low energy is different from high energies. This is due to early longitudinal collision dynamics. V 1 values lie on a common trend. STAR : PRL 92 (2004) 062301 PRL101 （ 2008 ） 252301 NA49: PRC68(2003)034903 STAR Preliminary

13. Jiayun Chen for STAR Collaboration – QNP2009 13 Summary  We found that proton v 1 is "collapsed". By comparing to anti-proton v 1, such collapse is found not due to mass effect. The comparison of the ratio of proton v 1 to that of pions, shows that the shadowing effect alone cannot explain the collapse of proton v 1. Our finding is consistent with “anti-flow” phenomena.  V 1 results from 9.2GeV are similar to those obtained at SPS from collisions at similar energies  Partonic collectivity at RHIC done! Test of local thermalization is the next step.  Ideal hydro limit: v 4 /v 2 2 data larger than ideal hydro prediction.