First observation of exclusive ϕ meson photoproduction in heavy-ion UPCs with the CMS detector

by JiaZhao Lin (Rice University)

Tuesday 29 Apr 2025, 11:00 → 13:00 US/Eastern

CFNS/Physics Library

A key objective of high-energy nuclear physics is to search for the onset of gluon saturation phenomena in the limit of extreme gluon densities. Photon-induced vector meson production in ultraperipheral heavy-ion collisions (UPCs) provides a unique and powerful probe of the gluon distribution in nuclei. The $\varphi$ meson, with its mass around 1 GeV, lies at the boundary between the perturbative and nonperturbative QCD regimes, making it uniquely suited to probe the transition between these two domains. However, the significant challenge of detecting extremely low transverse momentum (around 100 MeV) kaons from exclusive $\varphi$ meson decays has hindered the measurement of its production in UPCs for decades. In this seminar, we present the first observation and measurement of exclusive $\varphi$ meson photoproduction via the $K^{+}{K}^{-}$ decay channel in PbPb UPCs at a center-of-mass energy per nucleon pair of 5.36 TeV. The differential cross sections for exclusive $\varphi$ meson photoproduction are reported as a function of $\varphi$ rapidity. The results are compared to various theoretical models and provide new insights into the small-$x$ nuclear gluonic structure at a critical scale.