Systematics of the chemical freeze-out line in the high baryon density regime explored at SIS100

TL;DR

This study investigates the uncertainties in chemical freeze-out parameters at high baryon densities using UrQMD simulations, revealing dependencies on hadron species and equation of state, with implications for interpreting QCD phase diagram features.

Contribution

It provides a systematic analysis of chemical freeze-out fits at SIS100 energies, highlighting dependencies on hadron selection and EoS, and discusses implications for QCD phase diagram interpretations.

Findings

Fit quality remains good despite non-sharp freeze-out surface.

Including light nuclei and anti-protons affects temperature estimates.

Stiffer EoS increases the chemical potential.

Abstract

The systematic uncertainties of chemical freeze-out fits at SIS100 energies (Au+Au reactions at $\sqrt{s_{NN}}=3-5$ GeV) are studied using UrQMD simulations. Although hadron production in UrQMD does not occur on a sharp chemical freeze-out hyper-surface, the extracted fit quality is shown to be very good. The extracted chemical parameters depend on the selected hadron species as well as the underlying equation of state (EoS) of the matter. Including light nuclei and anti-protons in the fit increases the expected freeze-out temperature, while a stiffer EoS increases the obtained chemical potential. Similarly, the baryon densities extracted by the thermal fits depend on the choice of hadrons as well as the underlying equation of state. These results are important for the upcoming CBM@FAIR physics program and highlight that a degree of caution is advised when one relates the chemical freeze-out curve to features on the QCD phase diagram like the critical endpoint or a possible phase transition.