/ Experiment-Nucl arXiv:1105.4126

Measurement of direct photon $v_2$ in $\sqrt{s_{NN}}=200$~GeV Au$+$Au collisions

Pages: 6

Abstract: The second Fourier component v_2 of the azimuthal anisotropy with respect to the reaction plane has been measured for direct photons at midrapidity and transverse momentum (p_T) of 1.15 GeV/c in Au+Au collisions at sqr(s_NN)=200 GeV. Previous measurements of this quantity for hadrons with p_T < 6 GeV/c indicate that the medium behaves as 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 we find that for p_T > 4 GeV/c, the anisotropy for direct photons is consistent with zero and current uncertainties prevent a conclusive test of predicted small v_2 values from jet conversion and fragmentation photons. 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 significantly underpredict the observed v_2.

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