Search for the standard model Higgs boson decaying to a pair of $\tau$ leptons and produced in association with a W or a Z boson in proton-proton collisions at $\sqrt{s}=13$ TeV

A search for the standard model Higgs boson produced in association with a W or a Z boson decaying leptonically is performed using a data sample of proton-proton collisions collected at $\sqrt{s} = 13$ TeV by the CMS experiment at the LHC corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The Higgs boson is sought in its decay to a pair of $\tau$ leptons. A significance of 2.3 standard deviations is observed (1.0 expected) for a Higgs boson mass of 125 GeV. The signal strength, $\mu = 2.5 ^{+1.4} _{-1.3}$, is measured relative to the expectation for the standard model Higgs boson. These results are combined with a previous analysis performed on the same data set targeting the gluon fusion and vector boson fusion production modes with the Higgs boson decaying to a pair of $\tau$ leptons. The combined results provide increased sensitivity to the Higgs boson couplings to fermions and vector bosons, which are measured to be compatible with standard model predictions within one standard deviation.