Eur. Phys. J. C (2011) 71: 1699
https://doi.org/10.1140/epjc/s10052-011-1699-x

Regular Article - Theoretical Physics

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

¹ Department of Physics, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
² Helsinki Institute of Physics, University of Helsinki, P.O. Box 64, 00014, Helsinki, Finland

^* e-mail: tuomas.lappi@jyu.fi

Received: 27 April 2011
Revised: 14 June 2011
Published online: 30 July 2011

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 () in nucleus–nucleus collisions than it does for protons (), 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.