Multi-messenger heavy-ion collision physics

TL;DR

This paper presents a comprehensive hybrid model for relativistic heavy-ion collisions, incorporating all stages from initial state to hadronic afterburner, and explores how photon observables can reveal the collision dynamics and quark-gluon plasma formation.

Contribution

It introduces a detailed hybrid model including pre-equilibrium effects and demonstrates the use of photon observables to probe collision stages and small system quark-gluon plasma formation.

Findings

Pre-equilibrium phase significantly affects photon and hadron observables.

Photon spectra and flow can indicate chemical equilibration time.

Photon production signals quark-gluon plasma in small collision systems.

Abstract

This work studies the production of direct photons in relativistic nuclear collisions, along with the production of hadrons. Radiation from the very first instants to the final moments of the evolution is included. The hybrid model used here describes all stages of relativistic heavy-ion collisions. Chronologically, those are an initial state reflecting the collision of nuclei described within the Color Glass Condensate effective theory; a pre-equilibrium phase based on non-equilibrium linear response; relativistic viscous hydrodynamics, and a hadronic afterburner. The effect of the pre-equilibrium phase on both photonic and hadronic observables is highlighted for the first time. The potential of photon observables -- spectrum, differential elliptic and triangular flow -- to reveal the chemical equilibration time is studied. Finally, we consider "small collision systems", including proton+nucleus collisions and collisions of light nuclei, as probed by hadronic and electromagnetic observables. We demonstrate how photon production can signal the formation of quark-gluon plasma in such small systems.