Higgs Searches in the $H \to WW$ Decay Mode Using the ATLAS Detector

The prospects for Higgs boson searches in the $H ightarrow WW$ decay mode are investigated. Search strategies are presented for the channels ${H+0j}$ (${H ightarrow WW ightarrow e umu u}$), ${H+2j}$ (${H ightarrow WW ightarrow e umu u}$), and ${H+2j}$ (${H ightarrow WW ightarrowell u qq}$). Methods to extract the background normalization from data are discussed for all channels. With an integrated luminosity of 10 fb$^{-1}$, the $H+0j$, $H ightarrow WW ightarrow e umu u$ channel can reach a 5$sigma$ discovery for a Standard Model Higgs boson in the range $145<M_{H}<180$~GeV; its mass can be measured with a precision of about 2 GeV if $M_{H}$ is around 160 GeV, or with a precision of about 6 GeV if $M_{H}$ is close to 140 GeV. The $H+2j$, $H ightarrow WW ightarrow e umu u$ channel has a similar sensitivity, reaching the 5$sigma$ significance level for $150<M_{H}<180$~GeV and yielding a less precise mass determination. The statistical significance for $H+2j$, $H ightarrowell u qq$ peaks at about 350 GeV, where it is slightly below the 5$sigma$ level; the expected error on the mass determination at this mass is about 6 GeV. The combined sensitivity of the two $H ightarrow WW ightarrow e umu u$ channels is estimated, and a combined fit including the $H ightarrow ZZ ightarrow 4ell$ channel is presented. With 1 fb$^{-1}$ of integrated luminosity, the combined fit can exclude Standard Model Higgs bosons with masses above 145~GeV. At 10 fb$^{-1}$ the expected significance is above the 5$sigma$ level for $M_{H}>135$~GeV.