First measurement of the energy and Mandelstam-$|t|$ dependence of both coherent and incoherent J/$\psi$ photonuclear production

Abstract

A phenomenon, gluon saturation, is expected to emerge in quantum chromodynamics (QCD) at high energies, when gluon splitting and recombination processes reach a dynamic equilibrium. In heavy nuclei, this balance is expected to be achieved at lower energies than in protons, making lead--lead collisions at the LHC an ideal environment to probe the onset of saturation. The diffractive photoproduction of the $J/\psi$ vector meson provides an excellent tool to study this regime, as it directly probes the gluon distribution in the target. ALICE offers unique kinematic coverage of the photon--nucleon centre-of-mass energy, spanning from 20 to 800~GeV, corresponding to three orders of magnitude in Bjorken-$x$ from about $10^{-2}$ down to $10^{-5}$. This contribution presents the latest ALICE results on the energy dependence of coherent $J/\psi$ production, which is sensitive to the average gluon density, and on the energy and Mandelstam-$|t|$ dependence of incoherent production, which probes fluctuations of the gluon field at different spatial scales. These measurements provide new constraints for testing QCD models in the high-energy regime and advance our understanding of the gluonic structure of nuclei.