Equilibration and freeze-out of an expanding gas in a transport approach in a Friedmann-Robertson-Walker metric

TL;DR

This paper integrates a Friedmann-Robertson-Walker metric into the SMASH transport model, validating it against exact solutions and analyzing particle freeze-out in an expanding universe, with potential applications to heavy-ion collisions.

Contribution

It introduces the implementation of a FRW metric into the SMASH transport approach and validates it against exact solutions for massless and massive particles.

Findings

Excellent agreement with exact solutions for massless particles.

Freeze-out times depend on particle mass and cross sections.

Potential relevance to relativistic heavy-ion collision analysis.

Abstract

Motivated by a recent finding of an exact solution of the relativistic Boltzmann equation in a Friedmann-Robertson-Walker spacetime, we implement this metric into the newly developed transport approach Simulating Many Accelerated Strongly-interacting Hadrons (SMASH). We study the numerical solution of the transport equation and compare it to this exact solution for massless particles. We also compare a different initial condition, for which the transport equation can be independently solved numerically. Very nice agreement is observed in both cases. Having passed these checks for the SMASH code, we study a gas of massive particles within the same spacetime, where the particle decoupling is forced by the Hubble expansion. In this simple scenario we present an analysis of the freeze-out times, as function of the masses and cross sections of the particles. The results might be of interest for their potential application to relativistic heavy-ion collisions, for the characterization of the freeze-out process in terms of hadron properties.