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.