Temperatures and chemical potentials at kinetic freeze-out in relativistic heavy ion collisions from coarse grained transport simulations

TL;DR

This study uses coarse grained transport simulations to analyze the thermodynamic properties, such as temperature and chemical potential, during the kinetic freeze-out stage in relativistic heavy ion collisions across various energies.

Contribution

It introduces a detailed method to determine temperature and chemical potential at kinetic freeze-out using a coarse grained transport approach, highlighting the continuous nature of decoupling.

Findings

Kinetic freeze-out is a continuous process.

Temperature and chemical potential vary during decoupling.

Results differ from traditional single-point phase diagram analyses.

Abstract

Using the UrQMD/coarse graining approach we explore the kinetic freeze-out stage in central Au + Au collisions at various energies. These studies allow us to obtain detailed information on the thermodynamic properties (e.g. temperature and chemical potential) of the system during the kinetic decoupling stage. We explore five relevant collision energies in detail, ranging from $\sqrt{s_{NN}}=2.4\,\mathrm{GeV}$ (GSI-SIS) to $\sqrt{s_{NN}}=200\,\mathrm{GeV}$ (RHIC). By adopting a standard Hadron Resonance Gas equation of state, we determine the average temperature $\langle T \rangle$ and the average baryon chemical potential $\langle\mu_{\mathrm{B}}\rangle$ on the space-time hyper-surface of last interaction. The results highlight the nature of the kinetic freeze-out as a continuous process. This differential decoupling is an important aspect often missed when summarizing data as single points in the phase diagram as e.g. done in Blast-Wave fits. We compare the key properties of the system derived by using our approach with other models and we briefly review similarities and differences.