Observation of Coherent 𝜙(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 μ𝑏 −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.