Abstract
This Letter reports the observation of π-lepton-pair production in ultraperipheral lead-lead collisions Pb+PbβPbβ‘(πΎβ’πΎβπβ’π)β’Pb and constraints on the π-lepton anomalous magnetic moment ππ. The dataset corresponds to an integrated luminosity of 1.44ββnbβ1 of LHC Pb+Pb collisions at βπ NN=5.02ββTeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a π-lepton decay, an electron or charged-particle track(s) from the other π-lepton decay, little additional central-detector activity, and no forward neutrons. The πΎβ’πΎβπβ’π process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations and a signal strength of ππβ’π=1.0β’3+0.06
β0.05 assuming the standard model value for ππ. To measure ππ, a template fit to the muon transverse-momentum distribution from π-lepton candidates is performed, using a dimuon (πΎβ’πΎ βπβ’π) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for ππ is β0.057<ππ<0.024.