What can we learn from fluctuations of particle ratios?

TL;DR

This paper investigates how fluctuations in particle ratios, specifically $K/\pi$, can test statistical models of particle production in heavy ion collisions and determine the nature of the hadronic phase.

Contribution

It introduces an observable to identify the governing statistical model of freeze-out and assesses its behavior across different energies and conditions.

Findings

The observable should be consistent across RHIC and LHC energies under the grand-canonical model.

Deviations from this scaling indicate non-equilibrium or canonical effects.

The method can also probe the existence and duration of an interacting hadron gas phase.

Abstract

We explain how fluctuations of 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 variations of this scaling for 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.