Measurements of $J/\psi$ and the impact of coincident photon-induced processes in ultra-peripheral Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}=5.36$ TeV with the ATLAS detector

In ultra-relativistic heavy-ion collisions, the Lorentz-contracted electromagnetic fields of the ions generate intense quasi-real photon fluxes. These can interact with photons emitted by the oncoming nucleus, or with the nucleus itself, either directly in inelastic processes or diffractively via pomeron exchange. Diffractive photo-nuclear processes can produce exclusive vector mesons that are uniquely sensitive to the spatial and momentum structure of the nuclear parton distribution functions, as well as spatial fluctuations (hot spots). In Run 3, the ATLAS experiment utilized a low-multiplicity track trigger in heavy ions for the first time, allowing the collection of a large sample of events with a few low momentum tracks. A substantial fraction of these are sensitive to di-lepton decays of vector mesons like the $J/\\psi$. Results on $J/\\psi$ cross-section are presented using the widest continuous rapidity range available at the LHC. The high photon flux also enables the occurrence of multiple photon-induced processes in a single collision. This talk also presents the first measurement of coincident production of $\\gamma\\gamma \\rightarrow \\mu^{+}\\mu^{-}$ and a $\\rho$ meson in UPC Pb+Pb collisions at 5.02 TeV and 5.36 TeV with ATLAS. Scaling of the coincidence $\\rho$ meson production rate with neutron topologies and the di-muon system's properties, such as its mass, are also presented. These results provide tighter constraints on photon fluxes and nuclear charge form factors, as well as insights into nuclear gluon PDFs, beyond those from inclusive $\\rho$ meson photo-production.