Ambiguities in the hadro-chemical freeze-out of Au+Au collisions at SIS18 energies and how to resolve them

TL;DR

This paper investigates the ambiguity in thermal model fits of Au+Au collision data at SIS18 energies, revealing two degenerate solutions with different physical interpretations and emphasizing the importance of considering time evolution and resonance effects.

Contribution

It demonstrates the existence of two degenerate chemical freeze-out solutions in low-energy nuclear collisions and analyzes their physical implications using transport simulations and thermodynamic properties.

Findings

Two distinct freeze-out temperatures fit the data equally well.

The high temperature solution resembles a cooling fireball.

The low temperature solution shows unphysical heating and compression.

Abstract

The thermal fit to preliminary HADES data of Au+Au collisions at $\sqrt{s_{_{NN}}}=2.4$ GeV shows two degenerate solutions at $T\approx50$ MeV and $T\approx70$ MeV. The analysis of the same particle yields in a transport simulation of the UrQMD model yields the same features, i.e. two distinct temperatures for the chemical freeze-out. While both solutions yield the same number of hadrons after resonance decays, the feeddown contribution is very different for both cases. This highlights that two systems with different chemical composition can yield the same multiplicities after resonance decays. The nature of these two minima is further investigated by studying the time-dependent particle yields and extracted thermodynamic properties of the UrQMD model. It is confirmed, that the evolution of the high temperature solution resembles cooling and expansion of a hot and dense fireball. The low temperature solution displays an unphysical evolution: heating and compression of matter with a decrease of entropy. These results imply that the thermal model analysis of systems produced in low energy nuclear collisions is ambiguous but can be interpreted by taking also the time evolution and resonance contributions into account.