Observation of direct-photon collective flow in Au+Au collisions at √SNN = 200 GeV

PHENIX Collaboration

Research output: Contribution to journalArticlepeer-review


The second Fourier component v2 of the azimuthal anisotropy with respect to the reaction plane was measured for direct photons at midrapidity and transverse momentum (pT) of 1-13 GeV/c in Au+Au collisions at √SNN = 200 GeV. Previous measurements of this quantity for hadrons with pT < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for pT > 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 pT > 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 pT < 4 GeV/c region dominated by thermal photons, we find a substantial direct photon v2 comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region significantly underpredict the observed v2.

Original languageEnglish (US)
Article number122302
JournalPhysical review letters
Issue number12
StatePublished - Sep 19 2012

ASJC Scopus subject areas

  • General Physics and Astronomy


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