Observation of the gamma gamma -> tau tau process in Pb+Pb collisions and constraints on tau-lepton anomalous magnetic moment with the ATLAS detector at LHC.

This paper reports the observation of tau-lepton pair production in ultraperipheral lead-lead collisions, Pb(gamma) Pb(gamma) -> tau tau and constraints on the tau-lepton anomalous magnetic moment, a(tau). The dataset corresponds to an integrated luminosity of 1.44 nb^-1 of LHC Pb+Pb collisions at sqrt(s(NN))=5.02 TeV. Selected events contain one muon from a tau-lepton decay, an electron or charged particle track(s) from the other tau-lepton decay, little additional central-detector, little additional central-detector activity, and no forward neutrons. The gamma gamma -> tau tau process is observed in Pb+Pb collisions with a signal strength of mu(tautau) = 1.03 +0:06 -0.05. To measure a(tau), a template fit to the muon transverse-momentum distribution from tau-lepton candidates is performed, using a dimuon (gamma gamma -> mu mu) control sample to constrain systematic uncertainties. The observed 95% confidence-level intervals for a(tau) is (-0.057; 0.024).