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

TL;DR

This study measures the azimuthal anisotropy of direct photons in high-energy gold-gold collisions, revealing a significant flow signal at low transverse momentum that challenges existing thermal photon models.

Contribution

It provides the first measurement of direct photon v_2 at high energies, showing substantial flow at low p_T that is not predicted by current models.

Findings

Direct photons show substantial v_2 below 4 GeV/c.

At p_T > 4 GeV/c, direct photon v_2 is consistent with zero.

Thermal photon models underpredict observed v_2 at low p_T.

Abstract

The second Fourier component v_2 of the azimuthal anisotropy with respect to the reaction plane was measured for direct photons at midrapidity and transverse momentum (p_T) of 1--13 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 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, 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 significantly underpredict the observed v_2.