Observation and measurement of Higgs boson decays to WW$^*$ with the ATLAS detector

We report the observation of Higgs boson decays to $WW^{\ast}$ based on an excess over background of $6.1$ standard deviations in the dilepton final state, where the Standard Model expectation is $5.8$ standard deviations. Evidence for the vector-boson fusion (VBF) production process is obtained with a significance of 3.2 standard deviations. The results are obtained from a data sample corresponding to an integrated luminosity of $25\,\textrm{fb}^{-1}$ from $\sqrt{s}=7$ and 8 TeV $pp$ collisions recorded by the ATLAS detector at the LHC. For a Higgs boson mass of 125.36 GeV, the ratio of the measured value to the expected value of the total production cross section times branching fraction is $1.09^{+0.16}_{-0.15}\,\textrm{(stat)}^{+0.17}_{-0.14}\,\textrm{(syst)}$. The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are $1.02\pm 0.19\,\textrm{(stat)}^{+0.22}_{-0.18}\,\textrm{(syst)}$ and $1.27^{+0.44}_{-0.40}\,\textrm{(stat)}^{+0.30}_{-0.21}\,\textrm{(syst)}$, respectively. At $\sqrt{s}=8$ TeV, the total production cross sections are measured to be $\sigma(gg\to$ $H\rightarrow WW^\ast) = 4.6\pm0.9\,\textrm{(stat)}\,^{+0.8}_{-0.7}\,\textrm{(syst)}\textrm{pb}$ and $\sigma(\textrm{VBF}$ $H\rightarrow WW^\ast) = 0.51\,^{+0.17}_{-0.15}\,\textrm{(stat)}\,^{+0.13}_{-0.08}\,\textrm{(syst)}\textrm{pb}$. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.