Observation of τ lepton pair production in ultraperipheral Pb-Pb collisions at √sNN = 5.02 TeV

Abstract

We present an observation of photon-photon production of τ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404 μb−1 collected by the CMS experiment at a center-of-mass energy per nucleon pair of √sNN = 5.02 TeV. The γγ → τ+ τ− process is observed for τ+ τ− events with a muon and three charged hadrons in the final state. The measured fiducial cross section is σ (γγ → τ+ τ−) = 4.8 ± 0.6 (stat) ± 0.5 (syst) μb, where the second (third) term corresponds to the statistical (systematic) uncertainty in σ (γγ → τ+ τ−) in agreement with leading-order QED predictions. Using σ (γγ → τ+ τ−), we estimate a model-dependent value of the anomalous magnetic moment of the τ lepton of a τ = 0.00 1 + 0.055 − 0.089.