Observation of Direct-Photon Collective Flow in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$

Observation of Direct-Photon Collective Flow in $Au + Au$ Collisions at $\sqrt{s_{N N}} = 200 GeV$

A. Adare et al. (PHENIX Collaboration)

Phys. Rev. Lett. 109, 122302 – Published 19 September 2012

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 $\sqrt{s_{N N}} = 200 GeV$ . 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 $v_{2}$ .