Event Anisotropy in Au+Au Collisions at RHIC

Raimond Snellings (LBNL)For the STAR Collaboration

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Overview

• Introduction• First elliptic flow measurement at RHIC

• Elliptic flow versus centrality • Elliptic flow for identified particles versus pt

• Elliptic “flow” for charged particles with a pt > 2 GeV/c

• Summary

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Why Heavy-Ion Collisions?Why Elliptic Flow Measurements?

• Study the bulk properties of matter far from the ground state (“parton” matter)

• The pressure - The pressure gradient generates collective motion (flow) • Central collisions: radial flow• Peripheral collisions: radial flow

and anisotropic flow

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A Hydro Calculation of Elliptic Flow

P. Kolb, J. Sollfrank, and U. Heinz

Equal energy density lines

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v2(pt) for high pt particles

M. Gyulassy, I. Vitev and X.N. Wang, nucl-th/00012092

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Summary (elliptic flow)

• Rescattering• Converts space

anisotropy to momentum anisotropy

• Becomes more spherical• Self-quenching!thermalization at

Early time

v2

t (fm/c)

Zhang, Gyulassy, Ko, PL B455 (1999) 45

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The STAR Detector at RHIC

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A schematic view of v2

Almond shape overlap region in coordinate space

ε = 〈y2 − x2 〉

〈y2 + x2 〉φ2cos2 =v

Momentum spacex

y

pp

atan=φ

P. Jacobs and G. Cooper, nucl-ex/0008015

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Azimuthal-angle distribution versus reaction plane

• v2 increases from central to peripheral collisions

φ2cos2 =v

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Excitation function

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Centrality Selection

PRL 86, (2001) 402

nch = primary tracks in |η| < 0.75

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Charged particle v2 versus centrality

• Boxes show “initial spatial anisotropy” εscaled by 0.19-0.25

PRL 86, (2001) 402

|η| < 1.3

0.1 < pt < 2.0

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Sub Event Correlation

• Non-Flow Effects• Momentum

conservation• HBT,

Coulomb (final state)

• Resonance decays

• Jets

( )( )

φ⋅

φ⋅=Ψ

∑

∑−

iii

iii

1B,A2 2cosw

2sinwTan

21

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Different “sub event” methods

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Systematic errors

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v2(pt) for a thermal source

43

212 22

22

)(CeCCeCmv

pm

pm

+

+

+

−=λ

λ

Simple thermal sourcePasi Huovinen

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Charged particle and charged pion v2(pt) (minimum bias)

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Charged pion and proton + anti proton v2(pt) (minimum bias)

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A Hydro view of the world

• Hydro calculations: P. Huovinen, P. Kolb and U. Heinz

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Charged pion v2(pt) for different centralities

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Charged particle anisotropy 0 < pt < 4.5 GeV/c

• Only statistical errors

• Systematic error 10% - 20% for pt= 2 – 4.5 GeV/c

• More in the STAR high-pt talk (James Dunlop, PS2, this afternoon)

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Summary

• Mass dependence of v2(pt) shows a behavior in agreement with hydro calculations

• Large v2 is an indication of early thermalization• First time in Heavy-Ion Collisions a system

created which at low pt is in quantitativeagreement with hydrodynamic model predictions for v2 for mid-central collisions

• Around pt > 2 GeV/c the data starts to deviate from hydro. However, v2 stays large.

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The STAR Collaboration

Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino

U.S. Labs: Argonne, Lawrence Berkeley National Lab, Brookhaven National Lab

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 deRecherches SubatomiquesStrasbourg, SUBATECH -Nantes

Germany: Max Planck Institute – Munich, University of Frankfurt

Poland: Warsaw University, Warsaw University of Technology

Institutions: 36Collaborators: 415 Students: ~50

Spokesperson: John Harris