In this letter, we consider strong and electromagnetic (ultraperipheral) mechanisms in protonnucleus coherent diffraction at the LHC. We explicitly demonstrate the dominance of the latter and explain the CMS data on the forward rapidity gap distribution in pP b collisions at √ sNN = 8.16 TeV. In particular, we provide simple estimates, which give a good, semi-quantitative description of both magnitude and shape of the ∆η F distribution in the Pomeron-proton topology. We also make predictions for the proton-oxygen run.

In this letter, we argue that the STAR data on J/ψ photoproduction in d + Au UPCs is not sensitive to leading twist gluon nuclear shadowing, gluon saturation, or details of the deuteron wave function, but instead primarily probes elastic and possibly proton-dissociative J/ψ photoproduction on quasi-free nucleons. Analyzing the STAR detector capabilities for selection of UPC events, we explore the possibility that the contribution of proton-dissociative J/ψ photoproduction is negligibly small and obtain a very good description of the t dependence of the STAR total cross section.