Observation of $\gamma\gamma\to\tau\tau$ production in PbPb collisions with the CMS experiment

Ultraperipheral lead-lead collisions produce very large photon fluxes that permit fundamental quantum-mechanical processes to be observed and studied. The first measurement of $\tau$ lepton pair in ultraperipheral PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV with data collected by CMS during the LHC Run 2 will be presented. The study paves the way for the determination of the anomalous magnetic moment of the $\tau$ lepton, currently poorly constrained.