Fluctuations as probe of the QCD phase transition and freeze-out in heavy ion collisions at LHC and RHIC

TL;DR

This paper investigates higher order moments of net baryon number fluctuations to understand the QCD phase transition and freeze-out conditions in heavy ion collisions at LHC and RHIC, revealing their potential as probes of the transition region.

Contribution

It introduces the use of higher order fluctuation moments and O(4) scaling functions to analyze the QCD phase transition and freeze-out, highlighting deviations from hadron resonance gas model predictions.

Findings

Ratios of sixth to second and eighth to second moments change rapidly near the transition.

Higher order moments deviate from hadron resonance gas model predictions at vanishing chemical potential.

Sixth order moments remain negative at the chiral transition temperature, indicating proximity to the crossover line.

Abstract

We discuss the relevance of higher order moments of net baryon number fluctuations for the analysis of freeze-out and critical conditions in heavy ion collisions at LHC and RHIC. Using properties of O(4) scaling functions, we discuss the generic structure of these higher moments at vanishing baryon chemical potential and apply chiral model calculations to explore their properties at non-zero baryon chemical potential. We show that the ratios of the sixth to second and eighth to second order moments of the net baryon number fluctuations change rapidly in the transition region of the QCD phase diagram. Already at vanishing baryon chemical potential they deviate considerably from the predictions of the hadron resonance gas model which reproduce the second to fourth order moments of the net proton number fluctuations at RHIC. We point out that the sixth order moments of baryon number and electric charge fluctuations remain negative at the chiral transition temperature. Thus, they offer the possibility to probe the proximity of the thermal freeze-out to the crossover line.