Evidence for J/$ψ$ suppression in incoherent photonuclear production

Abstract: 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/$\psi$ vector meson provides a direct insight into the gluon composition of hadrons. The J/$\psi$ 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/$\psi$ mesons off lead ions, at $\sqrt{s_{\rm NN}} = 5.02$ TeV, for three Mandelstam-$t$ intervals. The energy dependence of the photonuclear cross section at the highest $|t|$ range measured, $(0.81< |t| <1.44)$ GeV$^2$, 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.