Search for the Standard Model Higgs boson in the $H\rightarrow W^{+}W^{-}\rightarrow\ell^{+}\nu\ell^{-}\bar{\nu}$ decay mode in proton-proton collisions at $\sqrt{s}=7$ TeV and $\sqrt{s}=8$ TeV with the ATLAS experiment

Modern particle physics research is dedicated to study the fundamental constituents of matter and their interactions. Scientific research findings on both theoretical and experimental sides during the past decades have been condensed in the Standard Model of particle physics. In this model, the interactions between fundamental particles are described by gauge fields and the exchange of corresponding gauge bosons. The Standard Model contains several such bosons, for example the massive and charged W bosons and a neutral Z boson, that have been observed experimentally. The simplest and most popular implementation of electroweak symmetry breaking to attribute mass to the W and Z bosons is the Higgs mechanism. This mechanism implies the existence of one additional particle, the Higgs boson, that is the only remaining particle of the Standard Model to be established experimentally. In July 2012, the discovery of a new neutral boson with a measured mass of about 126 GeV was announced by the ATLAS and CMS collaborations at the particle physics research laboratory CERN near Geneva. This thesis was carried out within the ATLAS collaboration and is dedicated to the search for the Standard Model Higgs boson in the decay channel $H\rightarrow W^{+}W^{-}\rightarrow\ell^{+}\nu\ell^{-}\bar{\nu}$. The analysis is based on datasets collected in proton-proton collisions in 2011 and 2012 by the ATLAS experiment at the Large Hadron Collider LHC at CERN. The studies performed in the course of this thesis consisted in the multi-stage selection of the $H\rightarrow W^{+}W^{-}\rightarrow\ell^{+}\nu\ell^{-}\bar{\nu}$ final states and the construction of control regions to validate the Monte Carlo simulation of the main background components in general and of the Standard Model WW process, the dominant background, in particular. An independent and complementary study to constrain the diboson background was performed focusing on the selection of a very pure WZ control region. This method allows for a direct comparison of the diboson kinematic distributions between data and Monte Carlo predictions and opens the possibility to further improve the understanding of the WW background. An additional emphasis was placed on a solid understanding of the electron identification capabilities at the ATLAS experiment. A set of electron shower shape distributions was extracted using $Z \rightarrow e^{+}e^{-}$ decays on the dataset taken in 2010 by the ATLAS experiment. This study served as an important basis for the improvement of the calorimeter simulations and contributed significantly to the optimisation of the electron identification.