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 σᶠidₒbs = 12.4 +3.8-3.1 fb. Constraints are set on the contributions to the anomalous magnetic moment (a_τ) and electric dipole moments (d_τ) of the τ lepton originating from potential effects of new physics on the γττ vertex: a_τ = 0.0009 -0.0031+0.0032 and d_τ < 2.9 × 10⁻¹⁷ $e\,$cm (95% confidence level), consistent with the standard model.