Observation and measurement of Higgs boson decays to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>W</mml:mi><mml:mi>W</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>with the ATLAS detector

We report the observation of Higgs boson decays to $W{W}^{*}$ 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 $25\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ 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 $1.0{9}_{\ensuremath{-}0.15}^{+0.16}{(\mathrm{stat})}_{\ensuremath{-}0.14}^{+0.17}(\text{syst})$. The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are $1.02\ifmmode\pm\else\textpm\fi{}0.19{(\mathrm{stat})}_{\ensuremath{-}0.18}^{+0.22}(\text{syst})$ and $1.2{7}_{\ensuremath{-}0.40}^{+0.44}{(\mathrm{stat})}_{\ensuremath{-}0.21}^{+0.30}(\text{syst})$, respectively. At $\sqrt{s}=8\text{ }\text{ }\mathrm{TeV}$, the total production cross sections are measured to be $\ensuremath{\sigma}(gg\ensuremath{\rightarrow}H\ensuremath{\rightarrow}W{W}^{*})=4.6\ifmmode\pm\else\textpm\fi{}0.9{(\mathrm{stat})}_{\ensuremath{-}0.7}^{+0.8}(\text{syst})\text{ }\text{ }\mathrm{pb}$ and $\ensuremath{\sigma}(\mathrm{V}\mathrm{B}\mathrm{F} H\ensuremath{\rightarrow}W{W}^{*})=0.5{1}_{\ensuremath{-}0.15}^{+0.17}{(\mathrm{stat})}_{\ensuremath{-}0.08}^{+0.13}(\text{syst})\text{ }\text{ }\mathrm{pb}$. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.

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