Bose--Einstein Condensation and Thermalization of the Quark Gluon Plasma

TL;DR

This paper investigates the early-time behavior of the quark-gluon plasma in heavy ion collisions, focusing on Bose--Einstein condensation, thermalization processes, and the system's strongly interacting fluid nature despite weak coupling.

Contribution

It provides a theoretical analysis of gluon overpopulation, Bose--Einstein condensate formation, and the role of elastic and inelastic processes in the thermalization of the quark-gluon plasma.

Findings

Overoccupied gluon system can form a transient Bose--Einstein condensate.

Elastic scatterings drive the system towards condensation, while inelastic processes influence gluon number.

The system remains anisotropic and behaves as a strongly interacting fluid during thermalization.

Abstract

In ultra-relativistic heavy ion collisions, the matter formed shortly after the collision is a dense, out of equilibrium, system of gluons characterized by a semi-hard momentum scale $Q_{\rm s}$. Simple power counting arguments indicate that this system is over-occupied: the gluon occupation number is parametrically large when compared to a system in thermal equilibrium with the same energy density. On short time scales, soft elastic scatterings tend to drive the system towards the formation of a Bose--Einstein condensate that contains a large fraction of the gluons while contributing little to the energy density. The lifetime and existence of this condensate depends on whether inelastic processes, that occur on the same time scale as the elastic ones, preferably increase or decrease the number of gluons. During this overpopulated stage, and all the way to thermalization, the system behaves as a strongly interacting fluid, even though the elementary coupling constant is small. We argue that while complete isotropization may never be reached, the system may yet evolve for a long time with a fixed anisotropy between average longitudinal and transverse momenta.