  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.
