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

TL;DR

This paper proposes that late quark chemical equilibration in a gluon-rich QCD medium explains the unexpectedly large elliptic flow of thermal photons observed in heavy-ion collisions.

Contribution

It introduces a model where slow chemical equilibration enhances photon elliptic flow, aligning theoretical predictions with experimental data.

Findings

Late quark production leads to increased photon elliptic flow.

Numerical estimates show significant enhancement due to chemical non-equilibrium.

Medium is initially gluon-rich, affecting photon production timing.

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.