We report the observation of Higgs boson decays to WW∗ 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 25fb⁻¹ from √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 (stat) +0.17 −0.14 (syst) . The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are 1.02±0.19(stat) +0.22 −0.18 (syst) and 1.27 +0.44 −0.40 (stat) +0.30 −0.21 (syst) , respectively. At √s=8 TeV, the total production cross sections are measured to be σ(gg→ H→WW ∗ )=4.6±0.9(stat) +0.8 −0.7 (syst)pb and σ(VBF H→WW ∗ )=0.51 +0.17 −0.15 (stat) +0.13 −0.08 (syst)pb . The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.
