Observation of Direct-Photon Collective Flow inAu+AuCollisions atsNN=200 GeV</mml:mi...

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 $1--12\text{ }\text{ }\mathrm{GeV}/c$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$. Previous measurements of this quantity for hadrons with ${p}_{T}<6\text{ }\text{ }\mathrm{GeV}/c$ indicate that the medium behaves like a nearly perfect fluid, while for ${p}_{T}>6\text{ }\text{ }\mathrm{GeV}/c$ 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 ${p}_{T}>4\text{ }\text{ }\mathrm{GeV}/c$ 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 ${p}_{T}<4\text{ }\text{ }\mathrm{GeV}/c$ 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 ${v}_{2}$.