Observation of $γγ$ $\to$ $ττ$ in proton-proton collisions and limits on the anomalous electromagnetic moments of the $τ$ lepton

Abstract: The production of a pair of $\tau$ leptons via photon-photon fusion, $\gamma\gamma$ $\to$ $\tau\tau$, 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 $\gamma\gamma$ $\to$ $\tau\tau$ is $\sigma^\text{fid}_\text{obs}$ = 12.4 $^{+3.8}_{-3.1}$ 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 $\lvert d\tau \rvert$ $\lt$ 2.9 $\times$ 10$^{-17}$ $e\,$cm (95% confidence level), consistent with the standard model.