Anisotropic flow from Au+Au collisions at GeV

1. Anisotropic flow from Au+Au collisions at GeV Aihong Tang for the Collaboration Aihong Tang, CIPANP 2003 New York

2. py px z initial spatial anisotropy anisotropy in momentum space y x isotropic directed elliptic Definitions: directed flow, elliptic flow RHIC : v2 ~ 6% v1 not yet measured Aihong Tang, CIPANP 2003 New York

3. Zhang, Gyulassy, Ko, PL B455 (1999) 45 Free-streaming does not convert spatial anisotropy to momentum anisotropy Developed early (t < 2 fm/c) Why is elliptic flow interesting? Sensitive to the physics of constituent interactions at early time Aihong Tang, CIPANP 2003 New York

4. M. Gyulassy, I. Vitev and X.N. Wang Why is Elliptic Flow at high pt interesting? Results from jet energy loss from different emission angles with respect to the reaction plane. Sensitive to the medium density profile Aihong Tang, CIPANP 2003 New York

5. Seen by an experimentalist Seen by theory Elliptic Flow - Experimental complications In experiment, vn can only be obtained from azimuthal correlations between outgoing particles. Some non-reaction plane related correlation among particles can confuse standard flow analysis. Aihong Tang, CIPANP 2003 New York

6. Nonflow effects - an extreme example Consider an “event” that is consist of co-moving particle pairs that are randomly distributed in azimuthparticles are not correlated with respect to the reaction plane (v2=0 is expected) Two particle correlation flow analyses give: Pure nonflow effect ! Nonflow: correlations that contribute to standard v2 but are unrelated to collective motion rel. to reaction plane… Examples include: Jets,Resonance decay,HBT effects,Final state interactions, etc. Aihong Tang, CIPANP 2003 New York

7. Elliptic Flow - Methods Particles correlated with the event plane Both are based on two-particle correlations Particles pair correlations Four-particle correlations (cumulants) Aihong Tang, CIPANP 2003 New York

8. Four particle correlations - motivations for cumulants The correlation between two particles is Correlating four particles, one gets The nonflow term is thus canceled in the cumulant Aihong Tang, CIPANP 2003 New York

9. Comparing 130 GeV and 200 GeV STAR Preliminary central peripheral Aihong Tang, CIPANP 2003 New York

10. Nucl-ex/0303001 Elliptic flow; reaching hydro limit? Aihong Tang, CIPANP 2003 New York

11. v2 versus pt at 200 GeV STAR Preliminary Significant v2 up to ~7 GeV/c in pt, the region where hard scattering begins to dominate. Aihong Tang, CIPANP 2003 New York

12. v2 from 2, 4 and 6 particle cumulant STAR Preliminary Considerable nonflow from 2 particle correlations. Nonflow from 4 particle correlation, v{6}-v{4} is negligible. Aihong Tang, CIPANP 2003 New York

13. peripheral Flow+nonflow in AuAu Nonflow in pp central STAR Preliminary Nonflow study at high pt Aihong Tang, CIPANP 2003 New York

14. Conclusions • v2 for central collisions at RHIC reaches hydro limit. • Nonflow contribution to 4 particle correlations is negligible. • Significant v2 up to 7 GeV/c in middle central events at 200 GeV. • Nonflow at high pt could be big in peripheral and central events. Aihong Tang, CIPANP 2003 New York

15. Back up slide Nonflow study at high pt Aihong Tang, CIPANP 2003 New York