Entropy Creation in Relativistic Heavy Ion Collisions

TL;DR

This paper reviews entropy production mechanisms in relativistic heavy ion collisions, including decoherence, viscous hydrodynamics, and strong coupling approaches, providing a comprehensive framework for understanding entropy growth from initial interactions to late stages.

Contribution

It introduces a unified framework based on the Husimi distribution for calculating entropy growth in quantum field theories relevant to heavy ion collisions, connecting weak and strong coupling regimes.

Findings

Entropy exhibits linear growth during the early glasma phase.

Husimi distribution allows entropy calculation from first principles.

Entropy production at strong coupling analyzed via AdS/CFT duality.

Abstract

We review current ideas on entropy production during the different stages of a relativistic nuclear collision. This includes recent results on decoherence entropy and the entropy produced during the hydrodynamic phase by viscous effects. We start by a discussion of decoherence caused by gluon bremsstrahlung in the very first interactions of gluons from the colliding nuclei. We then present a general framework, based on the Husimi distribution function, for the calculation of entropy growth in quantum field theories, which is applicable to the early ("glasma") phase of the collision during which most of the entropy is generated. The entropy calculated from the Husimi distribution exhibits linear growth when the quantum field contains unstable modes and the growth rateis asymptotically equal to the Kolmogorov-Sina\"i (KS) entropy. We outline how the approach can be used to investigate the problem of entropy production in a relativistic heavy-ion reaction from first principles. Finally we discuss some recent results on entropy production in the strong coupling limit, as obtained from AdS/CFT duality.