The first observation of coherent φð1020Þ 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 1.62 μb−1. The φð1020Þ meson signals are reconstructed via the KþK− decay channel. The production cross section is presented as a function of the φð1020Þ meson rapidity in the range 0.3 < jyj < 1.0, probing gluons that carry a fraction of the nucleon momentum (x) around 10−4. The observed cross section exhibits little dependence on rapidity and is significantly suppressed, by a factor of ∼5, 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 φð1020Þ data than those incorporating gluon saturation. This study establishes a powerful new tool for exploring nuclear effects and nuclear gluonic structure in the small-x regime at a unique energy scale bridging the perturbative and nonperturbative quantum chromodynamics domains
