Probing the neutron-skin thickness through J/ψ photoproduction in ultra-peripheral collisions

We study the impact of neutron-skin thickness on J/ψ photoproduction in ultra-peripheral ²⁰⁸Pb + ²⁰⁸Pb collisions. Within the Color Glass Condensate framework, we calculate coherent and incoherent cross sections and examine their dependence on the momentum transfer ∣t∣ for different neutron-skin thicknesses. We find a clear imprint of the neutron skin on the ∣t∣ spectra: a larger neutron skin leads to a smoother and more extended color-density profile, suppressing the coherent cross section at large ∣t∣ while enhancing the incoherent cross section through increased event-by-event configurational fluctuations in the nuclear periphery. We further show that the ratio of incoherent to coherent integrated cross sections provides a particularly sensitive and robust observable, with reduced theoretical uncertainties. These results establish diffractive vector-meson photoproduction in ultra-peripheral collisions as a powerful tomographic tool to constrain the neutron-skin thickness and the transverse gluon distribution at the LHC and future Electron-Ion Colliders.