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We present predictions for diffractive photoproduction of $\rho^0$ and $J/\psi$ vector mesons in ultra-peripheral oxygen–oxygen and neon–neon collisions using the energy-dependent hotspot model.
A striking prediction of this model is that the energy dependence of the incoherent cross section provides a clear signature of saturation effects: the cross section rises with energy, reaches a maximum, and decreases as saturation sets in. The position of this maximum depends on the mass of the produced vector meson and the mass number of the target. In our previous work, we demonstrated that there is a dependence on the Mandelstam-$t$ variable, with the maximum appearing at $|t|$ values corresponding to the size of subnucleonic hotspots. This model has been successfully compared to LHC data for coherent and incoherent $J/\psi$ production in Pb–Pb UPCs.
We extend this approach to a detailed study of photoproduction of $\rho^0$ and $J/\psi$ in O–O and Ne–Ne collisions, considering also two different nuclear shape models for each system: a traditional Woods–Saxon parameterization, and an alpha-cluster model for oxygen and a bowling-pin–like shape for neon. These measurements are experimentally feasible with data collected during the 2025 LHC runs.
These studies have been submitted for publication and posted in arxiv:2509.11359 [hep-ph].
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