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

Abstract

We study the transverse momentum spectrum and the elliptic flow ${v}_{2}$ of photons produced in Au+Au collisions at $\sqrt{{s}_{NN}}=200$ 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 (sQGP) [$q+\overline{q}\ensuremath{\rightarrow}g+\ensuremath{\gamma}$ and $q(\overline{q})+g\ensuremath{\rightarrow}q(\overline{q})+\ensuremath{\gamma}$], the decays of hadrons ($\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\gamma}+\ensuremath{\gamma}$, $\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}+\ensuremath{\gamma}$, $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\pi}+\ensuremath{\gamma}$, ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\rho}+\ensuremath{\gamma}$, $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{\eta}+\ensuremath{\gamma}$, ${a}_{1}\ensuremath{\rightarrow}\ensuremath{\pi}+\ensuremath{\gamma}$) as well as their interactions ($\ensuremath{\pi}+\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\rho}+\ensuremath{\gamma}$, $\ensuremath{\rho}+\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\pi}+\ensuremath{\gamma}$, meson-meson bremsstrahlung $m+m\ensuremath{\rightarrow}m+m+\ensuremath{\gamma}$). The PHSD calculations reproduce the transverse momentum spectrum, the effective temperature ${T}_{\mathrm{eff}}$ and the elliptic flow ${v}_{2}$ 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 ${v}_{2}$. We find that the large direct photon ${v}_{2}$---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 ${v}_{2}$ as in the PHENIX experiment. On the other hand the ${v}_{2}$ 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 ${v}_{2}$ 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 ${v}_{2}$ 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 an effective slope parameter ${T}_{\mathrm{eff}}$ far above the critical temperature for the deconfinement phase transition.