Observation of Direct-Photon Collective Flow in Au plus Au Collisions at root s(NN)=200 GeV

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2012-09-19
Authors
Adare, Andrew
Hill, John
Kempel, Todd
Lajoie, John
Lebedev, Alexandre
Ogilvie, Craig
Pei, H.
Rosati, Marzia
Semenov, Alexey
Vale, Carla
Wei, Feng
et al.
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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).

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

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