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The second Fourier component v_2 of the azimuthal anisotropy with respect tothe reaction plane was measured for direct photons at midrapidity andtransverse momentum (p_T) of 1--13 GeV/c in Au+Au collisions at sqr(s_NN)=200GeV. Previous measurements of this quantity for hadrons with p_T < 6 GeV/cindicate that the medium behaves like a nearly perfect fluid, while for p_T > 6GeV/c a reduced anisotropy is interpreted in terms of a path-length dependencefor parton energy loss. In this measurement with the PHENIX detector at theRelativistic Heavy Ion Collider we find that for p_T > 4 GeV/c the anisotropyfor direct photons is consistent with zero, as expected if the dominant sourceof direct photons is initial hard scattering. However, in the p_T < 4 GeV/cregion dominated by thermal photons, we find a substantial direct photon v_2comparable to that of hadrons, whereas model calculations for thermal photonsin this kinematic region significantly underpredict the observed v_2.

Observation of direct-photon collective flow in sqrt(s_NN)=200 GeV Au+Au collisions