Probing chemical freeze-out criteria in relativistic nuclear collisions with coarse grained transport simulations

TL;DR

This paper introduces a new method using transport simulations to determine the chemical freeze-out surface in relativistic nuclear collisions, confirming some criteria and limiting others across a wide energy range.

Contribution

The study develops a novel approach combining scattering rates and coarse-grained transport models to extract freeze-out conditions, providing a microscopic basis for existing criteria.

Findings

Chemical freeze-out coincides with <E>/<N> ≈ 1 GeV at all energies.

Certain freeze-out criteria are only valid at higher energies.

The method aligns microscopic simulations with statistical model analyses.

Abstract

We introduce a novel approach based on elastic and inelastic scattering rates to extract the hyper-surface of the chemical freeze-out from a hadronic transport model in the energy range from E$_\mathrm{lab}=1.23$ AGeV to $\sqrt{s_\mathrm{NN}}=62.4$ GeV. For this study, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model combined with a coarse-graining method is employed. The chemical freeze-out distribution is reconstructed from the pions through several decay and re-formation chains involving resonances and taking into account inelastic, pseudo-elastic and string excitation reactions. The extracted average temperature and baryon chemical potential are then compared to statistical model analysis. Finally we investigate various freeze-out criteria suggested in the literature. We confirm within this microscopic dynamical simulation, that the chemical freeze-out at all energies coincides with $\langle E\rangle/\langle N\rangle\approx1$ GeV, while other criteria, like $s/T^3=7$ and $n_\mathrm{B}+n_\mathrm{\bar{B}}\approx0.12$ fm$^{-3}$ are limited to higher collision energies.