Elliptic flow of thermal photons in chemically non-equilibrated QCD medium

Abstract

Heavy-ion experiments have recently revealed that azimuthal momentum anisotropy -- elliptic flow -- of direct photons from a QCD medium is a few times larger than hydrodynamic predictions. I present a possible explanation for the enhancement based on late quark chemical equilibration. The color glass theory indicates that the medium is initially gluon-rich. Thermal photons are then mainly produced after quarks are produced, at which point large flow anisotropy is developed. The numerical estimations indicate that slow chemical equilibration visibly enhances the elliptic flow of thermal photons.