Anisotropic emission of direct photons from Au+Au collisions at $\sqrt {s_{\rm NN}}=200~{\rm GeV}$ with EPOS3 and IP-Glasma

Abstract

The observed large elliptic flow ($v_2$) of direct photons in relativistic heavy ion collisions challenges us to explain. In this paper we consider only two sources of direct photons, prompt photons and thermal photons. Prompt photons are calculated with QCD to next leading order, which fits well the spectra at high transverse momentum ($p_{\rm t}$) region. Thermal photons are calculated with AMY rate in a 3+1D viscous hydrodynamical evolution of the collision systems. Two types of initial conditions, EPOS3 and IP-Glasma, are employed. Our results with IP-Glasma initial condition agree with results in published work in most cases, and the difference tells the regions effected by the absence of viscous correction in photon emission rate. Because both models are well constrained with hadron data, the hydrodynamic evolution of collision systems in the two models have many common. However, the transverse velocity of the collective motion are quite different. This makes a big difference in the anisotropic emission of direct photon emission. The observed large elliptic flow of direct photons is reproduced with EPOS3 hydrodynamic evolution.