Observation of the SM scalar boson decaying to a pair of $\tau$ leptons with the CMS experiment at the LHC

A search for a standard model scalar boson decaying into a pair of $\tau$ leptons is performed using events recorded in proton-proton collisions by the CMS experiment at the LHC in 2016. The dataset corresponds to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$ at a center-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$. The $\tau$ leptons decay semi-hadronically, or leptonically to an electron or a muon, and the four final states with the largest branching fractions are considered. An excess of events is observed over the expected background prediction with a significance of 4.9 standard deviations for the scalar boson mass $m_{H} = 125$ GeV, to be compared to an expected significance of 4.7 standard deviations. The best fit of the observed $H \to \tau \tau$ signal cross section times branching fraction for $m_{H}= 125$ GeV is $1.06^{+0.25}_{-0.24}$ times the standard model expectation.