Evidence for the Higgs Boson in the H$\rightarrow WW^{(*)}\rightarrow \ell\nu\ell\nu$ Decay Channel

Evidence for the Higgs boson is presented in the H$\rightarrow WW^{(*)}\rightarrow \ell\nu\ell\nu$ channel in the ATLAS detector with 20.3 $fb^{-1}$ of 8 TeV and 4.5 $fb^{-1}$ of 7 TeV proton-proton collisions at the Large Hadron Collider (LHC). The \hww\ analysis is one of the three Higgs boson decay channels used by the ATLAS collaboration in combination to discover the Higgs boson. As the LHC increased its instantaneous luminosity, the average number of collisions per proton bunch crossing, called pileup, was increased to around 20. The techniques presented in this thesis mitigate the effects of pileup such as interaction vertex matching for event level quantities such as the missing transverse momentum. Optimization procedures for lepton identification, which is crucial for the semi-leptonic W decays, are presented and methods to model fake leptons which are quarks and gluons imitating leptons from W and Z boson decays. Finally, performance projections for future LHC running conditions are explored to inform funding agencies. These techniques are applied to the gluon-gluon fusion (ggF) and vector boson fusion (VBF) production of the Higgs boson on the LHC. Both production modes were measured to be consistent with the Standard Model expectations.