Observation of coherent $\varphi$(1020) meson photoproduction in ultraperipheral PbPb collisions at $\sqrt{s_{\text{NN}}}$ = 5.36 TeV

Abstract

The first observation of coherent $\varphi$(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 $\mu$b${}^{-1}$. The $\varphi$(1020) meson signals are reconstructed via the K${}^{+}$K${}^{-}$ decay channel. The production cross section is presented as a function of the $\varphi$(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 $\sim$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 $\varphi$(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.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-24-009 (CMS Public Pages)

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