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 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 ffiffi ffi s p 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 AE 0.19stat 0.22 -0.18 syst and 1.27 0.44 -0.40 stat 0.30 -0.21 syst, respectively. At ffiffi ffi s p 8 TeV, the total production cross sections are measured to be gg H WW 4.6 AE 0.9stat 0.8 -0.7 syst pb and 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.