Photon elliptic flow in relativistic heavy-ion collisions: Hadronic versus partonic sources

Abstract

We study the transverse momentum spectrum and the elliptic flow v2 of photons produced in Au+Au collisions at sqrt(s)=200 GeV using the Parton-Hadron-String Dynamics (PHSD) transport approach. As sources for photon production, we incorporate the interactions of off-shell quarks and gluons in the strongly interacting quark-gluon plasma (q+qbar->g+gamma and q/qbar+g->q/qbar+gamma), the decays of hadrons (pi->gamma+gamma, eta->gamma+gamma, omega->pi+gamma, eta'->rho+gamma, phi->eta+gamma, a_1->pi+gamma) as well as their interactions (pi+pi->rho+gamma, rho+pi->pi+gamma, bremsstrahlung m+m->m+m+gamma). The PHSD calculations reproduce the transverse momentum spectrum, the effective temperature T_eff and the elliptic flow v2 of both inclusive and direct photons as measured by the PHENIX Collaboration. The photons produced in the QGP contribute slightly less then 50% to the observed spectrum, but have small v2. We find that the large direct photon v2 - comparable to that of hadrons - can be attributed to the intermediate hadronic scattering channels not subtracted from the data when following the same extraction procedure for v2 as in the PHENIX experiment. On the other hand the v2 of direct photons - as evaluated by the weighted average of direct photon channels - gives a lower signal. The difference between the two extraction procedures for the direct photon v2 can be attributed to different definitions for the yield ratio of direct photons to the background photons. The QGP phase causes the strong elliptic flow of photons indirectly, by enhancing the v2 of final hadrons due to the partonic interaction in terms of explicit parton collisions and the mean-field potentials. We also show that the presence of the QGP radiation is manifest in the slope of the direct photon spectrum, leading to a slope parameter T_eff far above the critical temperature for the deconfinement phase transition.