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 τ leptons via photon–photon fusion, γγ→ττ , 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 τ 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 τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section of γγ→ττ is σobsfid=12.4−3.1+3.8fb . Constraints are set on the contributions to the anomalous magnetic moment ( aτ ) and electric dipole moments ( dτ ) of the τ lepton originating from potential effects of new physics on the γττ vertex: aτ=0.0009−0.0031+0.0032 and |dτ|<2.9×10−17ecm (95% confidence level), consistent with the standard model.