Lattice QCD results on cumulant ratios at freeze-out

TL;DR

This paper compares lattice QCD calculations of cumulant ratios at freeze-out with experimental data, revealing deviations from the hadron resonance gas model and providing insights into the QCD equation of state at finite baryon chemical potential.

Contribution

The study presents lattice QCD results on higher-order cumulants and analyzes their implications for the applicability of HRG models at freeze-out.

Findings

Cumulant ratios deviate from HRG predictions at freeze-out.

Skewness and kurtosis ratios show μ_B dependence consistent with QCD calculations.

Limitations of HRG thermodynamics are identified based on cumulant comparisons.

Abstract

Ratios of cumulants of net proton-number fluctuations measured by the STAR Collaboration show strong deviations from a skellam distribution, which should describe thermal properties of cumulant ratios, if proton-number fluctuations are generated in equilibrium and a hadron resonance gas (HRG) model would provide a suitable description of thermodynamics at the freeze-out temperature. We present some results on sixth order cumulants entering the calculation of the QCD equation of state at non-zero values of the baryon chemical potential (mu_B) and discuss limitations on the applicability of HRG thermodynamics deduced from a comparison between QCD and HRG model calculations of cumulants of conserved charge fluctuations. We show that basic features of the $\mu_B$-dependence of skewness and kurtosis ratios of net proton-number fluctuations measured by the STAR Collaboration resemble those expected from a O(mu_B^2) QCD calculation of the corresponding net baryon-number cumulant ratios.