First global study of super dense gluonic matter with UPCs by ALICE

Abstract

ALICE was the first experimental collaboration observing a moderate nuclear suppression down to low Bjorken-x in lead nuclei using coherent J/ψ photoproduction. In this contribution, we present new results extending the studies of the photonuclear cross section by covering the Bjorken-x interval of 1.1 · 10−5 < x < 3.3 · 10−2, corresponding to the photon-nuclear energies 17 < WγPb,n < 920 GeV. This is achieved by using multiple methods to extract the energy dependence, including new results on the forward neutron emission accompanying the coherent photoproduction process. These new results, combined with ALICE measurements of J/ψ off a proton target, probe the gluonic structure of the lead nuclei at the lowest Bjorken-x possible with any current experiment, challenging both gluon saturation and shadowing models to describe the data.