Search for the associated production of the Higgs boson and a vector boson in proton-proton collisions at $\sqrt{s}=$ 13 TeV via Higgs boson decays to $\tau$ leptons

A search for the standard model Higgs boson produced in association with a W or a Z boson and decaying a pair of $\tau$ leptons is performed. A data sample of proton-proton collisions collected at $\sqrt{s}=$ 13 TeV by the CMS experiment at the CERN LHC is used, corresponding to an integrated luminosity of 35.9 fb${}^{-1}$. The signal strength is measured relative to the expectation for the standard model Higgs boson, yielding $\mu =$ 2.5 ${}_{-1.3}^{+1.4}$. These results are combined with earlier CMS measurements targeting Higgs boson decays to a pair of $\tau$ leptons, performed with the same data set in the gluon fusion and vector boson fusion production modes. The combined signal strength is $\mu =$ 1.24 ${}_{-0.27}^{+0.29}$ (1.00 ${}_{-0.23}^{+0.24}$ expected), and the observed significance is 5.5 standard deviations (4.8 expected) for a Higgs boson mass of 125 GeV.