Probing the Neutron-Skin Thickness Through <i>J</i> / <i>ψ</i> Photoproduction in Ultra-Peripheral Collisions

Abstract We study the impact of neutron-skin thickness on J / ψ photoproduction in ultra-peripheral 208 Pb+ 208 Pb 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.