Net-proton number cumulant ratios as function of beam energy from an expanding nonequilibrium chiral fluid

TL;DR

This paper models net-proton number fluctuations in heavy-ion collisions using a nonequilibrium chiral fluid dynamic approach, revealing how critical phenomena and phase transitions influence observable cumulant ratios across beam energies.

Contribution

It introduces a coupled fluid dynamic and Langevin equation model to connect chiral order parameter fluctuations with net-proton cumulants, accounting for nonequilibrium effects and phase transition dynamics.

Findings

Critical end point signals are visible in cumulant ratios across a range of energies.

Nonequilibrium first-order phase transition with mixed phase affects cumulant behavior.

Model provides a framework to interpret experimental net-proton fluctuation data.

Abstract

The beam energy scan program at RHIC provides data on net-proton number fluctuations with the goal to detect the QCD critical end point and first-order phase transition. Interpreting these experimental signals requires a vital understanding of the interplay of critical phenomena and the nonequilibrium dynamics of the rapidly expanding fireball. We study these aspects with a fluid dynamic expansion coupled to the explicit propagation of the chiral order parameter sigma via a Langevin equation. Assuming a sigma-proton coupling through an effective proton mass, we relate cumulants of the order parameter and the net-proton number at freeze-out and obtain observable cumulant ratios as a function of beam energy. We emphasize the role of the nonequilibrium first-order phase transition where a mixed phase with gradual freeze-out can significantly alter the cumulants. We find that the presence of a critical end point is clearly visible in the cumulant ratios for a relatively wide range of center-of-mass energies.