Jozso's Legacy: Chemical and Kinetic Freeze-out in Heavy-Ion Collisions

TL;DR

This paper reviews the theoretical framework of chemical and kinetic freeze-out in heavy-ion collisions, demonstrating that chemical freeze-out is likely driven by a phase transition, unlike kinetic freeze-out which depends on collision centrality.

Contribution

It establishes that chemical freeze-out is associated with a phase transition, providing a new interpretation of experimental data and predicting centrality dependence of kinetic freeze-out temperatures.

Findings

Kinetic freeze-out temperature depends on collision centrality.

Chemical freeze-out temperature shows no centrality dependence.

Chemical freeze-out likely corresponds to the quark-hadron phase transition.

Abstract

We review J. Zimanyi's key contributions to the theoretical understanding of dynamical freeze-out in nuclear collisions and their subsequent applications to ultra-relativistic heavy-ion collisions, leading to the discovery of a freeze-out hierarchy where chemical freeze-out of hadron yields precedes the thermal decoupling of their momentum spectra. Following Zimanyi's lines of reasoning we show that kinetic freeze-out necessarily leads to a dependence of the corresponding freeze-out temperature on collision centrality. This centrality dependence can be predicted within hydrodynamic models, and for Au+Au collisions at RHIC this prediction is shown to reproduce the experimentally observed centrality dependence of the thermal decoupling temperature, extracted from hadron momentum spectra. The fact that no such centrality dependence is observed for the chemical decoupling temperature, extracted from the hadron yields measured in these collisions, excludes a similar kinetic interpretation of the chemical decoupling process. We argue that the chemical decoupling data from Au+Au collisions at RHIC can only be consistently understood if the chemical freeze-out process is driven by a phase transition, and that the measured chemical decoupling temperature therefore measures the critical temperature of the quark-hadron phase transition. We propose additional experiments to further test this interpretation.