Probing the QCD phase diagram with fluctuations

TL;DR

This paper explores how higher order cumulants of conserved charges can be used to probe the QCD phase diagram, especially near the phase boundary, through theoretical models and lattice QCD results.

Contribution

It combines theoretical scaling functions, chiral model calculations, and lattice QCD data to analyze fluctuations and their potential to reveal critical phenomena in QCD.

Findings

Higher order cumulants show rapid variation at the phase boundary.

Ratios of susceptibilities reflect deconfinement transition characteristics.

Polyakov loop susceptibilities indicate deconfinement in lattice QCD.

Abstract

The relevance of higher order cumulants of conserved charges for the analysis of freeze-out and critical conditions in heavy ion collisions at LHC and RHIC is discussed. Using properties of $O(4)$ scaling functions, the generic structure of these higher cumulants at vanishing baryon chemical potential is discussed. Chiral model calculations are then used to study their properties at non-zero baryon chemical potential. It is argued that the rapid variation of sixth and higher order cumulants at the phase boundary may be used to explore the QCD phase diagram in experiment. Moreover, results for the Polyakov loop susceptibilities in SU(3) lattice gauge theory as well as in (2+1) flavor lattice QCD are discussed. An analysis of the ratios of susceptibilities indicates that the deconfinement transition is reflected in characteristic modifications of these ratios.