# Chemical freeze-out in relativistic heavy-ion collisions

## TL;DR

This paper explains why particle abundances in relativistic heavy-ion collisions appear to reach chemical equilibrium at higher temperatures than kinetic freeze-out, attributing it to constant entropy per particle during hadronic evolution.

## Contribution

It demonstrates that the apparent chemical equilibrium at higher temperatures is due to entropy conservation, supported by transport and resonance gas models.

## Key findings

- Particle abundances match chemical equilibrium at higher temperatures.
- Entropy per particle remains constant during hadronic evolution.
- Transport and resonance gas models confirm the entropy-based explanation.

## Abstract

One surprising result in relativistic heavy-ion collisions is that the abundance of various particles measured in experiments is consistent with the picture that they reach chemical equilibrium at a temperature much higher than the temperature they freeze out kinetically. Using a multiphase transport model to study particle production in these collisions, we find that the above result is due to the constancy of the entropy per particle during the evolution of the hadronic matter from the chemical to the kinetic freeze-out. We further use a hadron resonance gas model to illustrate the result from the transport model study.