1. STAR S.A. Voloshin Elliptic Flow at RHIC STAR Collaboration U.S. Labs: Argonne, Berkeley, Brookhaven National Labs U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale Brazil Universidade de Sao Paolo China IHEP - Beijing, IPP - Wuhan England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt Poland: Warsaw University Warsaw University of Technology Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP - Protvino

2. STAR S.A. Voloshin Geometry of STAR ZDC Barrel EM Calorimeter Endcap Calorimeter Magnet Coils TPC Endcap & MWPC ZDC FTPCs Vertex Position Detectors Central Trigger Barrel or TOF Time Projection Chamber Silicon Vertex Tracker RICH Year 1: Magnet, TPC, CTB, ZDC, RICH

3. STAR S.A. Voloshin STAR TPC cosmic ray event in the TPC  Active volume: Cylinder R=2 m, L=4 m 139,000 electronics channels 512 time buckets each

4. STAR S.A. Voloshin Au on Au at CM Energy ~ 130 AGeV Data Taken June 25, 2000. Pictures from online display.

5. STAR S.A. Voloshin Data set Minimum bias trigger - Zero Degree Calorimeter coincidence Magnetic field 0.25 tesla p t > 75 MeV/c 22k events with reconstructed vertex | Z vertex | < 75 cm, | X vertex | < 1 cm, | Y vertex | < 1 cm Quality cuts for flow analysis 0.1 < p t < 2.0 GeV/c -- to have efficiency constant +/- 10% |  | < 1.3

6. STAR S.A. Voloshin Vertex finder and tracking efficiencies Beam-gas interactions CTB trigger threshold Correlation between Central Trigger Barrel signal and number of primary tracks => near constant tracking efficiency Beam-gas interactions All events Reconstructed vertex CTB trigger threshold

7. STAR S.A. Voloshin Elliptic Flow Rescattering Converts space to momentum anisotropy Becomes more spherical Self-quenching t (fm/c) Zhang, Gyulassy, Ko, PL B455 (1999) 45 X Y Elliptic flow => Early time physics XZ-plane - the reaction plane

8. STAR S.A. Voloshin Azimuthal Anisotropy pXpX pYpY  i Event Plane - an estimator for the reaction plane Elliptic flow:

9. STAR S.A. Voloshin Method Summary Define subevents - independent groups of particles Correlate subevent planes Calculate the reaction plane resolution Correlate particles with a plane Gives v( , p t ) Correct for the reaction plane resolution

10. STAR S.A. Voloshin Quality Check: Some Details Particle  distribution in the lab TPC sector boundaries Event plane distribution Particle Correlation with respect to event plane Note: Highly suppressed zero (fluctuations ~3%)

11. STAR S.A. Voloshin n ch - number of primary tracks in |  | < 0.75 ~ 90% of all hadronic Au+Au interactions Eight centrality bins Centrality

12. STAR S.A. Voloshin The Signal Non-Flow Effects Momentum conservation HBT, Coulomb (final state) Resonance decays Jets (jet quenching -- flow !) 2nd harmonic 1st harmonic Subevents chosen Pseudorapidity Random Charge A A B B -1<  <-0.050.05<  <1 First and higher harmonics => systematic error for elliptic flow

13. STAR S.A. Voloshin Centrality Dependence  = initial space anisotropy =  y 2 - x 2  /  y 2 + x 2  Curve for v 2 /  = 0.16 Systematic error: 0.005 Uncertainty of 10% in  total => 5% in b/2R Subevents chosen: - Pseudorapidity - Random - Charge Particles correlated: - In opposite hemisphere - From all other particles - Opposite charge Cuts:

14. STAR S.A. Voloshin Minimum Bias pseudorapiditytransverse momentum (average over 8 centrality bins, weighted with n ch ) GeV/c

15. STAR S.A. Voloshin Summary v 2 increases with collision energy AGS (full energy) 2% SPS3.5% RHIC6% Comparison with theory Data: v 2,max  0.06 RQMD: v 2,max  0.025, different centrality dependence UrQMD: v 2,max  0.015 Data: v 2 /   0.16 - 0.17 Hydro: v 2 /   0.19 - 0.25

16. STAR S.A. Voloshin Conclusions Elliptic flow is large at RHIC, v 2,max  0.06 Reaction plane resolution is good for related studies v 2 (  ) constant for |  | < 1.3 v 2 (p t ) almost linear up to 2 GeV/c stronger than average radial in-plane expansion Centrality dependence close to hydrodynamic model Magnitude approaching hydrodynamic model prediction Consistent with significant early-time equilibration

17. STAR S.A. Voloshin  K p d e Anisotropic flow: Next Flow of identified particles Flow of high p t particles Directed flow

18. STAR S.A. Voloshin v 2 Theory RQMD v2.4Hydro: P.F. Kolb, et al RHIC 160 GeV/A SPS SPS 40 GeV/A b (fm) v 2 / 