Observation of γγ→ττ in proton–proton collisions and limits on the anomalous electromagnetic moments of the τ lepton

The production of a pair of tau leptons via photon-photon fusion, gamma gamma -> tau tau, is observed for the first time in proton-proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb(-1). Events with a pair of tau leptons produced via photon-photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The tau leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section of gamma gamma -> tau tau is sigma(fid)(obs) = 12.4(-3.1)(+3.8) fb. Constraints are set on the contributions to the anomalous magnetic moment (a(tau)) and electric dipole moments (d(tau)) of the tau lepton originating from potential effects of new physics on the gamma tau tau vertex: a(tau) = 0.0009(-0.0031)(+0.0032) and vertical bar d(tau vertical bar) < 2.9 x 10(-17) e cm (95% confidence level), consistent with the standard model.