Nuclear structure and saturation effects from diffractive vector meson production

We study exclusive vector meson production in ultraperipheral collisions (UPCs) of a wide range of nuclei, and assess the potential of measurements to constrain the small- x structure of oxygen and neon nuclei. We employ an impact-parameter-dependent color glass condensate framework incorporating Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, Kovner evolution, with parameters constrained by a recent global Bayesian analysis of γ + p and γ + Pb data. We present predictions for coherent and incoherent J / ψ production in O + O and Ne + Ne UPCs at LHC energies, and quantify theoretical uncertainties using posterior samples from the calibration. We employ several nuclear structure models and find that t -differential observables are sensitive to the chosen model. We further study the mass-number dependence of saturation effects through nuclear suppression factors for coherent and incoherent vector meson production. Saturation-induced suppression increases systematically with both nuclear mass number and energy. Our results provide a unified framework for the systematic study of the onset of gluon saturation and nuclear structure at high energy, accessible in future UPC measurements at the LHC and at the electron-ion collider.