Observation of the γγ → ττ Process in Pb + Pb Collisions and Constraints on the τ-Lepton Anomalous Magnetic Moment with the ATLAS Detector

This Letter reports the observation of Formula Presented-lepton-pair production in ultraperipheral lead-lead collisions Formula Presented and constraints on the Formula Presented-lepton anomalous magnetic moment Formula Presented. The dataset corresponds to an integrated luminosity of Formula Presented of LHC Formula Presented collisions at Formula Presented recorded by the ATLAS experiment in 2018. Selected events contain one muon from a Formula Presented-lepton decay, an electron or charged-particle track(s) from the other Formula Presented-lepton decay, little additional central-detector activity, and no forward neutrons. The Formula Presented process is observed in Formula Presented collisions with a significance exceeding 5 standard deviations and a signal strength of Formula Presented assuming the standard model value for Formula Presented. To measure Formula Presented, a template fit to the muon transverse-momentum distribution from Formula Presented-lepton candidates is performed, using a dimuon (Formula Presented) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for Formula Presented is Formula Presented. © 2023 CERN, for the ATLAS Collaboration.