Observation of Coherent (Formula presented) Meson Photoproduction in Ultraperipheral PbPb Collisions at (Formula presented)

The first observation of coherent (Formula presented) meson photoproduction off heavy nuclei is presented using ultraperipheral lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The data were collected by the CMS experiment and correspond to an integrated luminosity of (Formula presented). The (Formula presented) meson signals are reconstructed via the (Formula presented) decay channel. The production cross section is presented as a function of the (Formula presented) meson rapidity in the range (Formula presented), probing gluons that carry a fraction of the nucleon momentum ((Formula presented)) around (Formula presented). The observed cross section exhibits little dependence on rapidity and is significantly suppressed, by a factor of (Formula presented), compared to a baseline model that treats a nucleus as a collection of free nucleons. Theoretical models that incorporate the nuclear shadowing effect generally provide a better description of the (Formula presented) data than those incorporating gluon saturation. This study establishes a powerful new tool for exploring nuclear effects and nuclear gluonic structure in the small-(Formula presented) regime at a unique energy scale bridging the perturbative and nonperturbative quantum chromodynamics domains.