Measurement of (Formula presented) Meson Photoproduction in Ultraperipheral Heavy Ion Collisions

This Letter reports the first measurement of photonuclear (Formula presented) meson production in ultraperipheral heavy ion collisions. The study is performed using lead-lead collision data, with an integrated luminosity of (Formula presented), collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.36 TeV. Photonuclear events, where one of the colliding nuclei breaks up and the other remains intact, are selected based on breakup neutron emissions and by requiring no particle activity in a large rapidity interval in the direction of the photon-emitting nucleus. The (Formula presented) mesons are reconstructed via the (Formula presented) decay channel, with the cross section measured as a function of (Formula presented) meson transverse momentum and rapidity. The results are compared with next-to-leading-order perturbative QCD calculations that employ recent parametrizations of the lead nuclear parton distribution functions, as well as with predictions based on the color glass condensate framework. This measurement is the first photonuclear collision study characterizing parton distribution functions of lead nuclei for parton fractional momenta (Formula presented) (relative to the nucleon) ranging approximately from a few (Formula presented) to (Formula presented) for different hard energy scale (Formula presented) selections. © 2026 CERN, for the CMS Collaboration.