Particlization of an interacting hadron resonance gas with global conservation laws for event-by-event fluctuations in heavy-ion collisions

TL;DR

This paper develops a new method for simulating hadronization in heavy-ion collisions that accounts for global conservation laws and non-Poissonian fluctuations, improving the understanding of fluctuation measurements at the LHC.

Contribution

It introduces a partitioning approach for particlization that incorporates conservation laws and non-Poissonian fluctuations, enhancing the realism of heavy-ion collision simulations.

Findings

Global conservation laws significantly affect fluctuation measurements.

Net proton and net baryon cumulant ratios differ markedly, impacting data interpretation.

Experimental net-charge fluctuations are suppressed compared to hadronic model predictions.

Abstract

We revisit the problem of particlization of a QCD fluid into hadrons and resonances at the end of the fluid dynamical stage in relativistic heavy-ion collisions in a context of fluctuation measurements. The existing methods sample an ideal hadron resonance gas, therefore, they do not capture the non-Poissonian nature of the grand-canonical fluctuations, expected due to QCD dynamics such as the chiral transition or QCD critical point. We address the issue by partitioning the particlization hypersurface into locally grand-canonical fireballs populating the space-time rapidity axis that are constrained by global conservation laws. The procedure allows to quantify the effect of global conservation laws, volume fluctuations, thermal smearing and resonance decays on fluctuation measurements in various rapidity acceptances, and can be used in fluid dynamical simulations of heavy-ion collisions. As a first application, we study event-by-event fluctuations in heavy-ion collisions at the LHC using an excluded volume hadron resonance gas model matched to lattice QCD susceptibilities, with a focus on (pseudo)rapidity acceptance dependence of net baryon, net proton, and net charge cumulants. We point out large differences between net proton and net baryon cumulant ratios that make direct comparisons between the two unjustified. We observe that the existing experimental data on net-charge fluctuations at the LHC shows a strong suppression relative to a hadronic description.