Nonequilibrium effects in hadronic fireball expansion

TL;DR

This paper compares transport and hydrodynamic models of a hadronic fireball's expansion, revealing significant deviations from chemical equilibrium, especially for hyperons, due to nonequilibrium effects.

Contribution

It provides a comparative analysis of transport and hydrodynamic models for hadronic fireball expansion, highlighting nonequilibrium effects on hadron abundances.

Findings

Transport and hydrodynamic models predict different evolution of hadron densities.

Significant deviations from chemical equilibrium occur during expansion.

Hyperonic species show especially pronounced nonequilibrium effects.

Abstract

We consider a spherical volume of hot and dense hadronic matter (fireball) expanding into a vacuum. It is assumed that initially the fireball matter is in local thermal and chemical equilibrium with vanishing collective velocity. The time evolution of the fireball is studied in parallel within the GiBUU transport model and an ideal hydrodynamic model. The equation of state of an ideal hadronic gas is used in the hydrodynamic calculation. The same set of hadronic species is used in transport and fluid-dynamical simulations. Initial coordinates and momenta of hadrons in transport simulations have been randomly generated by using the Fermi and Bose distributions for (anti)baryons and mesons. The model results for radial profiles of densities and collective velocities of different hadronic species are compared at different times. We find that two considered models predict essential differences in time evolution of hadron abundances, which are especially pronounced for hyperonic species. This gives an evidence of a strong deviation from chemical equilibrium in expanding hadronic matter.