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 σ fid obs = 12.4 +3.8 −3.1 fb. 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+0.0032 −0.0031 and |dτ| < 2.9 × 10−17 e cm (95% confidence level), consistent with the standard model.
