Cumulants of the chiral order parameter at lower RHIC energies

TL;DR

This study investigates how cumulants of the chiral order parameter vary with beam energy in heavy-ion collisions, aiming to identify signals of the QCD critical end point and phase transition through nonequilibrium effects.

Contribution

It introduces a fluid dynamic model coupled with Langevin dynamics for the chiral order parameter to analyze cumulants as signals of phase transitions in QCD.

Findings

Cumulants show distinct patterns near the critical end point.

Multiple crossings of the freeze-out line enhance cumulant signals.

Volume fluctuations significantly affect cumulant measurements.

Abstract

We study cumulants of the chiral order parameter as function of beam energy as a possible signal for the presence of a critical end point and first-order phase transition in the QCD phase diagram. We model the expansion of a heavy-ion collision by a fluid dynamic expansion coupled to the explicit propagation of the chiral order parameter sigma via a Langevin equation. We evolve the medium until a parametrized freeze-out condition is met where we calculate event-by-event fluctuations and cumulants of sigma which are expected to follow the trend of net-proton number cumulants. We emphasize the role of a nonequilibrium first-order phase transition: The presence of an unstable phase causes the well-known bending of the trajectories in the space of temperature and baryochemical potential. For these cases at lower beam energies, the system crosses the freeze-out line more than once, allowing us to calculate a range of cumulants for each initial condition which are overall enhanced for the second hit of the freeze-out line. We thus find not only the critical end point but also the phase transition of the underlying model clearly reflected in the cumulants. The impact of volume fluctuations is demonstrated to play a measurable role for fluid dynamical evolutions that last significantly long.