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 ffiffiffiffiffiffiffiffi
sNN
p 1⁄4 5.02 TeV. The γγ → τ þ τ −
process is observed for τ þ τ − events with a muon and three charged hadrons in the final state. The measured
fiducial cross section is σðγγ → τ þ τ − Þ 1⁄4 4.8 % 0.6ðstatÞ % 0.5ðsystÞ μb, where the second (third) term
corresponds to the statistical (systematic) uncertainty in σðγγ → τ þ τ − Þ in agreement with leading-order
QED predictions. Using σðγγ → τ þ τ − Þ, we estimate a model-dependent value of the anomalous magnetic
moment of the τ lepton of a τ 1⁄4 0.001 þ0.055
−0.089 .
