Pre-equilibrium photons from the early stages of heavy-ion collisions

TL;DR

This paper uses QCD kinetic theory to calculate photon production during the early, pre-equilibrium phase of heavy-ion collisions, providing a scalable model that smoothly transitions to thermal photon production and can be integrated into existing simulations.

Contribution

It introduces a detailed comparison of pre-equilibrium and thermal photon rates, derives a simple scaling form for the photon spectrum, and provides a publicly available implementation for modeling early-stage photon production.

Findings

Photon spectrum follows a simple scaling with shear viscosity and entropy density.

Numerical simulations confirm analytical scaling predictions.

The model enables smooth transition from pre-equilibrium to thermal photon production.

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 do a detailed comparison of pre-equilibrium photon rates to the thermal photon production. We show that the photon spectrum radiated from a hydrodynamic attractor evolution satisfies a simple scaling form in terms of the specific shear viscosity $\eta/s$ and entropy density $dS/d\zeta \sim {\scriptstyle \left(T\tau^{1/3}\right)^{3/2}}_\infty$. 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. Finally, our publicly available implementation can be straightforwardly added to existing heavy ion models.