Chemical Freeze-Out and Higher Order Multiplicity Moments

TL;DR

This paper investigates higher order multiplicity moments in the hadron resonance gas model, revealing non-monotonic behaviors at finite chemical potential that characterize the chemical freeze-out conditions, supported by lattice QCD insights.

Contribution

It introduces a novel universal freeze-out condition based on the behavior of kurtosis and its products, linking moments to freeze-out parameters in the HRG model.

Findings

Non-monotonic behavior of skewness and kurtosis at finite chemical potential.

Identification of a universal freeze-out condition using kurtosis and susceptibility.

Support from lattice QCD studies for the proposed freeze-out criteria.

Abstract

We calculate the non-normalized moments of the particle multiplicity within the framework of the hadron resonance gas (HRG) model. At finite chemical potential $\mu$, a non-monotonic behavior is observed in the thermal evolution of third order moment (skewness $S$) and the higher order ones as well. Among others, this observation likely reflects dynamical fluctuations and strong correlations. The signatures of non-monotonicity in the normalized fourth order moment (kurtosis $\kappa$) and its products get very clear. Based on these findings, we introduce a novel condition characterizing the universal freeze-out curve. The chemical freeze-out parameters $T$ and $\mu$ are described by vanishing $\kappa\, \sigma^2$ or equivalently $m_4=3\,\chi^2$, where $\sigma$, $\chi$ and $m_4$ are the standard deviation, susceptibility and fourth order moment, respectively. The fact that the HRG model is not able to release information about criticality related to the confinement and chiral dynamics should not veil the observations related to the chemical freeze-out. Recent lattice QCD studies strongly advocate the main conclusion of the present paper.