Particle multiplicities in the central region of high-energy collisions from $k_T$-factorization with running coupling corrections

Abstract

Horowitz and Kovchegov have derived a $k_T$-factorization formula for particle production at small $x$ which includes running coupling corrections. We perform a first numerical analysis to confront the theory with data on the energy and centrality dependence of particle multiplicities at midrapidity in high-energy p+A (and A+A) collisions. Moreover, we point out a strikingly different dependence of the multiplicity per participant on $N_\text{part}$ in p+Pb vs.\ Pb+Pb collisions at LHC energies, and argue that the observed behavior follows rather naturally from the convolution of the gluon distributions of an asymmetric vs. symmetric projectile and target.