Observation of Coherent $\phi(1020)$ Meson Photoproduction in Ultraperipheral PbPb Collisions at $\sqrt{s_{NN}}$ = 5.36 TeV

The first observation of coherent ϕ ( 1020 ) 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 μ b − 1 . The ϕ ( 1020 ) meson signals are reconstructed via the K + K − decay channel. The production cross section is presented as a function of the ϕ ( 1020 ) meson rapidity in the range 0.3 < | y | < 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 ϕ ( 1020 ) 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.