Evidence for J/ψ Suppression in Incoherent Photonuclear Production

According to quantum chromodynamics, at sufficiently high energy, the structure of hadrons reveals a dynamic equilibrium between gluon splitting and gluon recombination - a phenomenon known as saturation. The process of diffractive photonuclear production of a J/ψ vector meson provides a direct insight into the gluon composition of hadrons. The J/ψ production as a function of momentum transferred in the interaction, quantified by the Mandelstam-t variable, serves as an excellent probe for studying the structure of hadrons within the impact-parameter plane, because different ranges in t are sensitive to the dynamics of the gluon field at varying spatial size scales. The ALICE Collaboration has measured the energy dependence of incoherent photonuclear production of J/ψ mesons off lead ions, at sNN=5.02 TeV, for three Mandelstam-t intervals. The energy dependence of the photonuclear cross section at the highest |t| range measured, (0.81<|t|<1.44) GeV2, is sensitive to subnucleonic structures of the Pb target. The increase of the cross section with energy at large |t| shows evidence of suppression with respect to the increase seen at low |t|. The observed pattern of the energy evolution in data is similar to that of gluon saturation models.