Search for a Higgs Boson Produced in Association with a W Boson at ATLAS

The Large Hadron Collider at CERN is the most modern proton-proton collider and data taking will start in 2009, with a centre-of-mass energy of 7 TeV. The ATLAS detector, which is one of two multi-purpose detectors at the Large Hadron Collider, is able to detect a Standard Model Higgs boson if it exists. This is one of the main tasks of the ATLAS experiment. This thesis deals with a Standard Model Higgs boson produced in association with a W boson. The Monte Carlo study is based on physics events generated at the nominal centre-of-mass energy of the Large Hadron Collider of 14 TeV. Large parts of this analysis have been done using the global Grid infrastructure of the Large Hadron Collider experiments. A mass range of the Higgs boson of mH = 130 - 190 GeV has been taken into account. In this mass range, the Higgs boson dominantly decays into a pair of W bosons, leading to initially three W bosons: WH -> WWW. Two orthogonal analysis channels have been investigated in detailed studies of the background properties. The first channel considers the leptonic decay of two W bosons, such that the leptons are of opposite charge. The third W boson then decays hadronically. The analysis is based on one-dimensional cuts, where the best cuts found are strict cuts on the transverse momenta of the leptons, a cut on the invariant mass of the jets, as well as a cut on the transverse jet momenta and the missing transverse energy. The second decay channel studied is dedicated to the leptonic decay of all three W bosons. Again, cuts on the transverse momenta of the leptons and the jets have been proven to be efficient, as well as the use of the spatial correlation of the decay products of the Higgs boson. The invariant mass of the leptons with opposite sign has been emerged as a very efficient cut to reject dominant diboson background contributions. The discovery reach of both channels separately as well as the combination has been calculated using Bayesian methods. The discovery reach is at maximum for a mass range of mH = 150 - 170 GeV, with a peak for mH = 170 GeV at 5 sigma. All results are scaled to an integrated luminosity of L = 30 fb^-1, which corresponds to approximately three years of data-taking at the design luminosity of 10^33 cm^-2 s^-1. The associated WH production improves the discovery reach for a Standard Model Higgs boson at the ATLAS detector and would also be useful for precision measurements of the couplings of the Higgs boson.