Comparison of chemical freeze-out criteria in heavy-ion collisions

TLDR

Particle yields in heavy‑ion collisions exhibit a striking regularity across a wide energy range, enabling simple parameter‑based models to describe them from 1 A GeV to 200 A GeV. This study systematically compares these various freeze‑out criteria. We examine fixed‑energy‑per‑particle, baryon‑plus‑antibaryon density, normalized entropy density, and percolation models, contrasting their predictions with recent thermal‑statistical freeze‑out parameters and assessing parameter sensitivity. All criteria reproduce yields above the AGS top energy, but at low energies the constant‑energy‑per‑particle condition best fits the data and is least sensitive to model assumptions.

Abstract

One of the most remarkable results to emerge from heavy-ion collisions over the past two decades is the striking regularity shown by particle yields at all energies. This has led to several very successful proposals describing particle yields over a very wide range of beam energies, reaching from $1A$ GeV up to 200A GeV, using only one or two parameters. A systematic comparison of these proposals is presented here. The conditions of fixed energy per particle, baryon+anti-baryon density, normalized entropy density as well as percolation model are investigated. The results are compared with the most recent chemical freeze-out parameters obtained in the thermal-statistical analysis of particle yields. The sensitivity and dependence of the results on parameters is analyzed and discussed. It is shown that in the energy range above the top energy of the BNL Alternating Gradient Synchrotron within present accuracies, all chemical freeze-out criteria give a fairly good description of the particle yields. However, the low energy heavy-ion data favor the constant energy per particle as a unified condition of chemical particle freeze-out. This condition also shows the weakest sensitivity on model assumptions and parameters.