Microscopic models and effective equation of state in nuclear collisions at FAIR energies

TL;DR

This study uses microscopic models to analyze the formation of equilibrated nuclear matter in heavy-ion collisions at FAIR energies, revealing the equation of state and phase transition indicators.

Contribution

It introduces a detailed comparison of microscopic models with thermal models to determine equilibrium states and derives an effective equation of state with a transition point at 40 AGeV.

Findings

Nearly achieved equilibrium at all energies after relaxation.

Equation of state is linear with energy, with a change at 40 AGeV.

Kinks in phase diagrams indicate chemical freeze-out.

Abstract

Two microscopic models, UrQMD and QGSM, were employed to study the formation of locally equilibrated hot and dense nuclear matter in heavy-ion collisions at energies from 11.6 AGeV to 160 AGeV. Analysis was performed for the fixed central cubic cell of volume V = 125 fm**3 and for the expanding cell which followed the growth of the central area with uniformly distributed energy. To decide whether or not the equilibrium was reached, results of the microscopic calculations were compared to that of the statistical thermal model. Both dynamical models indicate that the state of kinetic, thermal and chemical equilibrium is nearly approached at any bombarding energy after a certain relaxation period. The higher the energy, the shorter the relaxation time. Equation of state has a simple linear dependence P = a(sqrt{s})*e, where a = c_s**2 is the sound velocity squared. It varies from 0.12 \pm 0.01 at E_{lab} = 11.6 AGeV to 0.145 \pm 0.005 at E_{lab} = 160 AGeV. Change of the slope in a(sqrt{s}) behavior occurs at E_{lab} = 40 AGeV and can be assigned to the transition from baryon-rich to meson-dominated matter. The phase diagrams in the T - mu_B plane show the presence of kinks along the lines of constant entropy per baryon. These kinks are linked to the inelastic (i.e. chemical) freeze-out in the system.