Observation of τ Lepton Pair Production in Ultraperipheral Pb-Pb Collisions at Formula Presented

We present an observation of photon-photon production of Formula Presented lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of Formula Presented collected by the CMS experiment at a center-of-mass energy per nucleon pair of Formula Presented. The Formula Presented process is observed for Formula Presented events with a muon and three charged hadrons in the final state. The measured fiducial cross section is Formula Presented, where the second (third) term corresponds to the statistical (systematic) uncertainty in Formula Presented in agreement with leading-order QED predictions. Using Formula Presented, we estimate a model-dependent value of the anomalous magnetic moment of the Formula Presented lepton of Formula Presented.