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 𝜏-lepton-pair production in ultraperipheral lead-lead collisions Pb+Pb→Pb⁡(𝛾⁢𝛾→𝜏⁢𝜏)⁢Pb and constraints on the 𝜏-lepton anomalous magnetic moment 𝑎𝜏. The dataset corresponds to an integrated luminosity of 1.44  nb⁻¹ of LHC Pb+Pb collisions at √𝑠NN = 5.02  TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a 𝜏-lepton decay, an electron or charged-particle track(s) from the other 𝜏-lepton decay, little additional central-detector activity, and no forward neutrons. The 𝛾⁢𝛾→𝜏⁢𝜏 process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations and a signal strength of 𝜇𝜏⁢𝜏 = 1.0⁢3^+0.06_−0.05 assuming the standard model value for 𝑎𝜏. To measure 𝑎𝜏, a template fit to the muon transverse-momentum distribution from 𝜏-lepton candidates is performed, using a dimuon (𝛾⁢𝛾→𝜇⁢𝜇) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for 𝑎𝜏 is −0.057<𝑎𝜏<0.024.