Observation and Measurement of Higgs Boson Decays to WW* with the ATLAS Detector

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 25 fb⁻¹ from √s̅ = 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{-0.15}^{+0.16}(stat){-0.14}^{0.17}(syst). The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are 1.02 ± 0.19(stat){-018}^{+0.22}(syst) and 1.27{-0.40}^{+0.44}(stat){-0.21}^{+0.30}(syst), respectively. At √s̅ = 8 TeV, the total production cross sections are measured to be σ(gg → H → WW*) = 4.6 ± 0.9(stat){-0.7}^{+0.8}(syst) pb and σ(VBF H → WW*) = 0.51{-0.15}^{+0.17}(stat){-0.08}^{+0.13}(syst) pb. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.