Measurement of coincident photon-initiated processes in ultra-peripheral Pb+Pb collisions with the ATLAS detector

Abstract: The Lorentz-contracted electromagnetic fields of the ions in ultra-relativistic heavy-ion collisions generate intense quasi-real photon fluxes. These lead to photon-induced interactions that are observed in ultra-peripheral collisions (UPCs), such as vector meson and lepton-pair production. The high photon flux also enables the occurrence of multiple photon-induced processes in a single collision. Presented is the first measurement of the coincident production of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ and $\gamma+A\rightarrow\rho^{0}+A$ in UPC Pb+Pb collisions at centre-of-mass energies of 5.02 TeV and 5.36 TeV with the ATLAS detector at the Large Hadron Collider. The rate of the coincident process relative to the exclusive $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ process is measured differentially in intervals of forward event activity, quantified by the Zero Degree Calorimeters. The relative rate, summed over forward event activity, for the coincident $\rho^{0}$ production is measured to be $(9.3\,\pm0.4\,\mathrm{(stat.)}\,\pm0.2\,\mathrm{(syst.)})\times10^{-3}$. Correlations between the dimuon kinematic properties, such as its mass, and the coincident $\rho^{0}$ meson production rate, are also presented. These measurements confirm the presence of multi photon-induced processes in UPC collisions, and can provide new insight into the impact parameter dependence of photon-induced vector meson production.