Thermal photons and collective flow at energies available at the BNL Relativistic Heavy-Ion Collider

Abstract

We update our calculations of thermal-photon production in nuclear collisions at the BNL Relativistic Heavy-Ion Collider (RHIC). Specifically, we address the recent experimental observation of an elliptic flow of direct photons comparable in magnitude to that of pions, which is at variance with expectations based on quark-gluon plasma (QGP) dominated photon radiation. Our thermal emission rate is based on previous work, i.e., resummed leading-order QGP emission and in-medium hadronic rates in the confined phase. These rates are nearly degenerate at temperatures close to the expected QCD-phase change. The rates are convoluted over an improved elliptic-fireball expansion with transverse- and elliptic-flow fields quantitatively constrained by empirical light- and strange-hadron spectra. The resulting direct-photon spectra in central Au-Au collisions are characterized by hadron-dominated emission up to transverse momenta of $\ensuremath{\sim}2$--$3\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$. The associated large elliptic flow in the hadronic phase mitigates the discrepancy with the measured photon-${v}_{2}$ compared to scenarios with QGP-dominated emission.