Campus Units

Physics and Astronomy

Document Type

Article

Publication Version

Published Version

Publication Date

9-19-2012

Journal or Book Title

Physical Review Letters

Volume

109

Issue

12

First Page

122302

DOI

10.1103/PhysRevLett.109.122302

Abstract

The second Fourier component v(2) of the azimuthal anisotropy with respect to the reaction plane is measured for direct photons at midrapidity and transverse momentum (p(T)) of 1-12 GeV/c in Au + Au collisions at root s(NN) = 200 GeV. Previous measurements of this quantity for hadrons with p(T) < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for p(T) > 6 GeV/c a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for p(T) > 4 GeV/c the anisotropy for direct photons is consistent with zero, which is as expected if the dominant source of direct photons is initial hard scattering. However, in the p(T) < 4 GeV/c region dominated by thermal photons, we find a substantial direct-photon v(2) comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region underpredict the observed v(2).

Comments

This is an article from Physical Review Letters 109 (2012):122302-1, doi:10.1103/PhysRevLett.109.122302. Posted with permission.

Copyright Owner

American Physical Society

Language

en

File Format

application/pdf

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