Observation and measurement of the Higgs boson in the WW∗ decay channel with ATLAS at the LHC

A measurement of the Higgs boson production has been performed with the AT- LAS detector at the Large Hadron Collider. Proton-proton collision data was used, corresponding to an integrated luminosity of 25 fb−1, obtained at center-of-mass energies of 7 and 8 TeV, during 2011 and 2012. The H → WW∗ → lνlν decay channel was analyzed, with particular emphasis on events with two electrons or two muons in the final state. New analysis techniques were developed to deal with the overwhelming contamination from Drell-Yan background processes. An overall excess of events was observed over the predicted background, with a signif- icance of 6.1σ, conclusively establishing the observation of the Higgs boson in the H → WW∗ → lνlν channel. The observed rate of events is compatible with the expectation for a Standard Model Higgs boson with a mass of 125.36 GeV, as shown by the ratio between the observed and expected cross section times branching ratio, measured at $1.08^{+0.16}_{-0.15} (stat.) ^{+0.08}_{-0.07} (expt.) ^{+0.13}_{-0.11} (theo.) \pm 0.03 (lumi.) = 1.08 ^{+0.22}_{−0.20}$