Relaxation to equilibrium and EOS in ultra-relativistic heavy-ion collisions

E Zabrodin,L Bravina

doi:10.1051/epjconf/201819105005

Abstract

We study relaxation to equilibrium of hot and dense hadron-string matter produced in the central zone of central heavy-ion collisions at energies 11:6AGeV ≤ Elab ≤ 160AGeV. Two microscopic transport models, UrQMD and QGSM, are employed. The analysis is performed for the central cubic cell with volume V = 125 fm3. To check how close the system is to the equilibrium, its hadron yields and hadron energy spectra are compared with those of the statistical model of ideal hadron gas. For all collision energies it was found that the matter in the cell was approaching the equilibrium state, which lasted about 10 - 20 fm/c. After that the matter became very dilute and the thermal contact between the hadrons was lost. Equation of state is well fitted to linear dependence P/ε = a = c2s, where the square of the sonic velocity c2s increases from 0.12 at Elab = 11:6AGeV to 0.145 at Elab = 160AGeV. These results are valid also for very early times of the system evolution when the matter is still out of equilibrium. Together with the isentropic expansion, the linear dependence of P on ε supports the application of hydrodynamic description to early stages of heavy-ion collisions.

Highlights

Hypothesis of local or even global equilibrium emerging in a system of particles produced in hadronic or nuclear collisions atrelativistic energies was utilized by Fermi in [1]
There is a big group of Monte Carlo microscopic transport models which do not rely on the assumption of local or global equilibrium
Two different Monte Carlo transport models, ultra-relativistic quantum molecular dynamics (UrQMD) and quarkgluon string model (QGSM), show that the hot and dense matter produced in the central area of central heavy-ion collisions at 11.6AGeV ≤ Elab ≤ 160 AGeV reaches the state which is very close to the state of thermal and chemical equilibrium

Summary

Introduction

Hypothesis of local or even global equilibrium emerging in a system of particles produced in hadronic or nuclear collisions at (ultra)relativistic energies was utilized by Fermi in [1]. There is a big group of Monte Carlo microscopic transport models which do not rely on the assumption of local or global equilibrium These models are successfully used to describe relativistic hadronic and heavy-ion collisions. Whether or not the system is in the vicinity of equilibrium, we have to compare the hadron abundances and energy spectra to those provided by the statistical model of ideal hadron gas with essentially the same degrees of freedom (i.e., hadronic species) as in the tested microscopic model To do this one has to define a certain volume inside the expanding fireball and extract energy density ε, net baryon density ρB, and net strangeness density ρS.

Basic features of microscopic transport models

Statistical model of ideal hadron gas

Relaxation to equilibrium and EOS

Findings

Conclusions