## Abstract

A measurement is presented of the $\mathrm {Z}/\gamma ^{*} \rightarrow \tau \tau$ cross section in $pp$ collisions at $\sqrt{s} = 13\hbox { TeV}$, using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of $2.3 fb^{−1}$. 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 $τ$ lepton production. The measurement also provides the reconstruction efficiency and energy scale for $τ$ decays to hadrons $+ν_τ$ final states, determined with respective relative uncertainties of 2.2 and 0.9%.