Pre-equilibrium photon production in QCD Kinetic Theory

Abstract

We use QCD kinetic theory to compute photon production in the chemically equilibrating Quark-Gluon Plasma created in the early stages of high-energy heavy-ion collisions. We show that the photon spectrum radiated from an attractor evolution satisfies a simple scaling form in terms of the specific shear viscosity $\eta /s$ and entropy density $dS/d\zeta \sim {{\left(T{\tau }^{1/3}\right)}^{3/2}}_{\infty }$. We confirm the analytical predictions with numerical kinetic theory simulations. We use the extracted scaling function to compute the pre-equilibrium photon contribution in $\sqrt{s_{NN}}=2.76\text{TeV}$ 0-20% PbPb collisions. We demonstrate that our matching procedure allows for a smooth switching from pre-equilibrium kinetic to thermal hydrodynamic photon production.

Comments: 6 pages, 2 figures; Proceeding on 11th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probe 2023), 26-31 March 2023, Aschaffenburg, Germany