Forward $J/\psi$ production at high energy: centrality dependence and mean transverse momentum

TL;DR

This paper investigates forward $J/$ meson production in proton-nucleus collisions, analyzing its dependence on collision centrality and transverse momentum, and compares theoretical predictions with experimental data.

Contribution

It extends previous dipole model calculations to include mean transverse momentum and centrality dependence, and explores backward rapidity using nuclear PDFs.

Findings

Overall good agreement with experimental data

Tendency for too strong centrality dependence

Small transverse area of small-$x$ gluons affects results

Abstract

Forward rapidity $J/\psi$ meson production in proton-nucleus collisions can be an important constraint of descriptions of the small-$x$ nuclear wavefunction. In an earlier work we studied this process using a dipole cross section satisfying the Balitsky-Kovchegov equation, fit to HERA inclusive data and consistently extrapolated to the nuclear case using a standard Woods-Saxon distribution. In this paper we present further calculations of these cross sections, studying the mean transverse momentum of the meson and the dependence on collision centrality. We also extend the calculation to backward rapidities using nuclear parton distribution functions. We show that the parametrization is overall rather consistent with the available experimental data. However, there is a tendency towards a too strong centrality dependence. This can be traced back to the rather small transverse area occupied by small-$x$ gluons in the nucleon that is seen in the HERA data, compared to the total inelastic nucleon-nucleon cross section.