Constant Trace Anomaly as a Universal Condition for the Chemical Freeze-Out

TL;DR

This paper proposes a new universal condition for chemical freeze-out in heavy-ion collisions based on a constant trace anomaly in the hadron resonance gas model, aligning well with previous conditions and suggesting minimal impact of hard-core interactions.

Contribution

It introduces the constant trace anomaly as a universal freeze-out condition and analyzes various HRG extensions to validate its effectiveness.

Findings

Constant trace anomaly aligns with previous freeze-out conditions.

Hard-core radii are negligible when all resonances below 2 GeV are included.

HRG with all resonances matches lattice QCD thermodynamics well.

Abstract

Finding out universal conditions describing the freeze-out parameters was a subject of various phenomenological studies. In the present work, we introduce a new condition based on constant trace anomaly (or interaction measure) calculated in the hadron resonance gas (HRG) model. Various extensions to the {\it ideal} HRG which are conjectured to take into consideration different types of interactions have been analysed. When comparing HRG thermodynamics to that of lattice quantum chromodynamics, we conclude that the hard-core radii are practically irrelevant, especially when HRG includes all resonances with masses less than $2~$GeV. It is found that the constant trace anomaly (or interaction measure) agrees well with most of previous conditions.