Measurements of J/ψ and impact of coincident photon induced processes in ultra-peripheral Pb+Pb collisions at √s_NN = 5.36 TeV with the ATLAS detector

AGH University of Krakow, Poland

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Published online: 17 April 2026

Abstract

The fully stripped ions used in heavy ion collisions at the LHC are an excellent source of high-energy quasi-real photons. These can interact with photons emitted by the oncoming nucleus, or with the nucleus itself, either directly in inelastic processes or diffractively via pomeron exchange. Diffractive photonuclear processes can produce exclusive vector mesons that are uniquely sensitive to the spatial and momentum structure of the nuclear parton distribution functions, as well as spatial fluctuations (hotspots). In Run 3, the ATLAS experiment utilized a low-multiplicity track trigger in heavy ions for the first time, allowing the collection of a large sample of events with a few tracks. A substantial fraction of these are sensitive to dilepton decays of vector mesons, including the J/ψ. First results on coherent J/ψ yields are presented using the widest continuous rapidity range available at the LHC. A discussion of important contributions from multiple-UPC processes is also provided.