Emergent hydrodynamics in microscopic modeling of early stage of relativistic heavy-ion collisions

L V Bravina,E E Zabrodin

doi:10.1140/epja/s10050-020-00265-y

Abstract

Time evolution of hot and dense nuclear matter produced in central gold-gold collisions at energies between E_{lab} = 10 and 160 AGeV is studied within two transport string models, UrQMD and QGSM. In contrast to the previous studies, here we investigate the macroscopic characteristics of the system before the state of chemical and thermal equilibrium is attained. For all energies in question two interesting observations are made for times starting already from t ge 1 fm/c. (1) The matter in the cell expands almost isentropically with nearly constant entropy per baryon. (2) Pressure in the cell appears to be very close to the pressure calculated for equilibrated hadron gas with the same values of energy density, baryon density and strangeness density. The pressure linearly depends on the energy density, P = a(sqrt{s}) varepsilon . Therefore, both observations endorses the formal application of relativistic hydrodynamics from the very early stages of heavy-ion collisions, despite of the fact that the matter in the fireball is out of equilibrium.

Highlights

Hydrodynamic, or rather hybrid, models are common tools nowadays for the description of heavy-ion and hadronnucleus collisions at relativistic energies, for review see [1,2] and references therein
Of primary interest is the evolution of energy density, pressure gradients, entropy per baryon, as well as comparison of these parameters with those calculated for the ideal hadron gas in chemical and thermal equilibrium
If the system of hadrons in microscopic model calculations is in the vicinity of equilibrium, its partial particle abundances and particle energy spectra have to be close to those given by the statistical model

Summary

Introduction

Hydrodynamic, or rather hybrid, models are common tools nowadays for the description of heavy-ion and hadronnucleus collisions at relativistic energies, for review see [1,2] and references therein. All models employ the assumption of local thermal equilibrium in the expanding fireball It appears that the thermalization time needed to match the experimental data is very short, e.g. τ ≈ 0.1 ÷ 0.5 fm/c. Relaxation of hot and dense matter, produced in the central area of relativistic heavy-ion collisions, to local equilibrium in microscopic model calculations was studied in [16,17,18,19,20,21,22,23,24,25,26]. Of primary interest is the evolution of energy density, pressure gradients, entropy per baryon, as well as comparison of these parameters with those calculated for the ideal hadron gas in chemical and thermal equilibrium.

Similarities and differences between the microscopic models

Basic principles of statistical model

Early stage of heavy-ion collisions

Conclusions