Fluctuations of particle ratios as a freeze-out probe

TL;DR

This paper proposes using particle ratio fluctuations as a probe to test statistical models of particle production and freeze-out conditions in heavy ion collisions, providing a method to distinguish between different theoretical scenarios.

Contribution

It introduces an observable based on $K/\pi$ fluctuations to determine the applicability of the Grand-Canonical statistical model and chemical equilibrium in heavy ion collisions.

Findings

The observable should be consistent across RHIC and LHC energies if the model holds.

Deviations indicate non-equilibrium or canonical effects.

The method can also assess the presence and duration of an interacting hadron gas phase.

Abstract

We explain how event-by-event fluctuations of particle ratios can constrain and falsify the statistical model of particle production in heavy ion collisions, using $K/\pi$ fluctuations as an example. We define an observable capable of determining which statistical model, if any, governs freeze-out in ultrarelativistic heavy ion collisions. We calculate this observable for $K/\pi$ fluctuations, and show that it should be the same for RHIC and LHC energies, as well as independent of centrality, if the Grand-Canonical statistical model is an appropriate description and chemical equilibrium applies. We describe what happens in case of deviations from this scenario, such as light quark chemical non-equilibrium, strange quark over-saturation and local conservation (canonical ensemble) for strange quarks. We also introduce a similar observable capable, together with the published $K^*/K$ measurement, of ascertaining if an interacting hadron gas phase governs the system between thermal and chemical freeze-out, and of ascertaining its duration and impact on hadronic chemistry