The hunt for the Higgs boson: the $WW \rightarrow \ell\nu\ell\nu$ final state at the ATLAS detector

Electroweak symmetry breaking stands as the last sector of the standard model of particle physics to be experimentally verified. The electroweak symmetry must be broken to incorporate massive particles into the theory and accurately reflect particle properties. The search for the Higgs boson is the search for the defining prediction of the Higgs mechanism, the simplest method to break electroweak symmetry. This thesis presents results on the search for the standard model Higgs boson in the $WW\rightarrow\ell\nu\ell\nu$ final state using 4.7 $fb^{-1}$~of data collected by the ATLAS detector in the 2011 data-taking at the LHC. The search is performed using a cut-based analysis to define the final signal regions in three lepton flavor channels ($ee$,$e\mu$,$\mu\mu$) and three jet multiplicities (0, 1, and $\geq$ 2). Backgrounds are estimated using several Monte Carlo simulation and data-driven techniques. The estimate of the $Z$+jets background is presented in particular detail, with three independent methods serving as comparisons to each other as well as studies focusing on particular aspects of the estimates. Fits of the final distributions of transverse mass are used for limit-setting in the $CL_{s}$ method. The mass range of 100 $< m_{H} <$ 600 GeV was searched for evidence of the existence of the standard model Higgs boson. No significant excesses above the prediction of known standard model processes was identified. Limits were then set at the 95\% confidence level excluding masses for the Higgs boson in the range 131 $< m_{H} <$ 260 GeV.