Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions

TL;DR

This paper investigates how the energy dependence of the saturation scale affects charged multiplicity in proton-proton and nucleus-nucleus collisions, highlighting the role of DGLAP evolution within the Color Glass Condensate framework.

Contribution

It explains the differing energy growth of charged multiplicity in pp and AA collisions through DGLAP evolution effects on the saturation scale, aligning with gluon saturation data from HERA.

Findings

Charged multiplicity grows faster in AA than in pp collisions.

DGLAP evolution influences the saturation scale's energy dependence.

Results are consistent with gluon saturation observations at HERA.

Abstract

A natural framework to understand the energy dependence of bulk observables from lower energy experiments to the LHC is provided by the Color Glass Condensate, which leads to a "geometrical scaling" in terms of an energy dependent saturation scale Q_s. The measured charged multiplicity, however, seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling. We argue that this difference between pp and AA collisions can be understood from the effect of DGLAP evolution on the value of the saturation scale, and is consistent with gluon saturation observations at HERA.