Measurement of the $\mathrm{Z}\gamma^{*} \to \tau\tau$ cross section in pp collisions at $\sqrt{s} = $ 13 TeV and validation of $\tau$ lepton analysis techniques

A measurement is presented of the $\mathrm {Z}/\gamma ^{*} \rightarrow \tau \tau$ cross section in $\text {pp}$ collisions at $\sqrt{s} = 13\hbox { TeV}$ , using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of $2.3\hbox { fb}^{-1}$ . The product of the inclusive cross section and branching fraction is measured to be $\sigma (\text {pp} \rightarrow \mathrm {Z}/\gamma ^{*}\text {+X}) \, \mathcal {B}(\mathrm {Z}/\gamma ^{*} \rightarrow \tau \tau ) = 1848 \pm 12\,(\text {stat}) \pm 67\text { (syst \,+\,lumi)}\text { pb}$ , in agreement with the standard model expectation, computed at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics. The measurement is used to validate new analysis techniques relevant for future measurements of $\tau$ lepton production. The measurement also provides the reconstruction efficiency and energy scale for $\tau$ decays to hadrons $\,+\,\nu _{\tau }$ final states, determined with respective relative uncertainties of 2.2 and 0.9%.