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^-1from √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.