High Gluon Densities in Heavy Ions Collisions

TL;DR

This paper reviews the concept of gluon saturation in high-energy heavy ion collisions, discussing the theoretical framework, challenges, and potential observable effects of dense gluon systems at small momentum fractions.

Contribution

It provides a pedagogical and critical overview of the challenges and progress in understanding gluon saturation and high-density gluon systems in heavy ion collisions.

Findings

Saturation tames gluon density growth at high energies.

The saturation momentum scale emerges dynamically and indicates non-linear gluon interactions.

High gluon density effects could be observable in heavy ion collision experiments.

Abstract

The early stages of heavy ion collisions are dominated by high density systems of gluons that carry each a small fraction $x$ of the momenta of the colliding nucleons. A distinguishing feature of such systems is the phenomenon of "saturation" which tames the expected growth of the gluon density as the energy of the collision increases. The onset of saturation occurs at a particular transverse momentum scale, the "saturation momentum", that emerges dynamically and that marks the onset of non-linear gluon interactions. At high energy, and for large nuclei, the saturation momentum is large compared to the typical hadronic scale, making high density gluons amenable to a description with weak coupling techniques. This paper reviews some of the challenges faced in the study of such dense systems of small $x$ gluons, and of the progress made in addressing them. The focus is on conceptual issues, and the presentation is both pedagogical, and critical. Examples where high gluon density could play a visible role in heavy ion collisions are briefly discussed at the end, for illustration purpose.