Search for a High-Mass Higgs Boson with the ATLAS Detector at the LHC

This thesis presents the search for a high-mass Higgs boson in the $H \to W^+ W^- \to \ell^+ \nu \ell^- \bar{\nu}$ decay channel. The final state with one electron and one muon and their associated neutrinos is considered. This signature is experimentally more sensitive than the one with two electrons or two muons, due to the strong suppression of background contributions from the production of a Z boson. For the production modes of the Higgs boson, the gluon-gluon fusion and vectorboson fusion channels are taken into account. Proton-proton collision data recorded by the LHC experiment ATLAS at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 13.2 fb$^{−1}$, are analysed. The spectrum of the transverse mass, reconstructed from the electron-muon pair and missing transverse momentum from the neutrinos, is probed for a signal excess above the background from Standard Model processes. Three signal regions are defined to target the characteristic kinematic properties of the two production channels, in order to discriminate the signal from the Standard Model background. Data control regions are used to constrain the normalisation of the dominant background contributions coming from top quark processes and the non-resonant decay of W boson pairs in a likelihood fit. Two signal interpretations are studied for a heavy boson mass up to 3000 GeV. The narrow width approximation assumes a negligible line width of the hypothesised boson, while the large width assumption considers widths up to 15% of the heavy boson mass. No significant excess above the Standard Model background is observed in the analysed data. In the absence of a signal excess, upper exclusion limits are placed at 95% confidence level on the production cross section of a charge neutral heavy scalar boson, multiplied with the branching ratio to two W bosons. For the interpretation of the narrow width approximation in the gluon-gluon fusion production mode, a cross section down to 1.37 and 0.051 pb is excluded for a mass of 400 and 2800 GeV, respectively. Upper limits on the cross section of a heavy boson with negligible width in the vectorboson fusion channel can be placed at 0.49 and 0.030 pb at the same mass points. For a heavy boson in the gluon-gluon fusion channel with a line width of 15% of its mass, a cross section down to 1.4 and 0.071 pb is excluded for a mass of 400 and 2800 GeV, respectively.