Mutual boosting of the saturation scales in colliding nuclei

TL;DR

This paper investigates how gluon saturation scales are mutually enhanced in nucleus-nucleus collisions due to small-x gluon saturation effects, leading to significantly increased saturation scales compared to proton-nucleus collisions.

Contribution

It derives reciprocity equations for the saturation scales in colliding nuclei and demonstrates their solutions show substantial enhancement in heavy ion collisions.

Findings

Saturation of small-x gluons causes transverse momentum broadening.

Gluon distribution is suppressed at large x and enhanced at small x.

Saturation scales are significantly boosted in nucleus-nucleus collisions.

Abstract

Saturation of small-x gluons in a nucleus, which has the form of transverse momentum broadening of projectile gluons in pA collisions in the nuclear rest frame, leads to a modification of the parton distribution functions in the beam compared with pp collisions. The DGLAP driven gluon distribution turns out to be suppressed at large x, but significantly enhanced at x<<1. This is a high twist effect. In the case of nucleus-nucleus collisions all participating nucleons on both sides get enriched in gluon density at small x, which leads to a further boosting of the saturation scale. We derive reciprocity equations for the saturation scales corresponding to a collision of two nuclei. The solution of these equations for central collisions of two heavy nuclei demonstrate a significant, up to several times, enhancement of Q_{sA}^2, in AA compared with pA collisions.