A Search for $H \to WW$ using a Matrix Element Discriminant and a WW Cross Section Measurement at ATLAS

One of the main motivating factors for the construction of the Large Hadron Collider (LHC) was the search for the Higgs boson, postulated to explain the origin of fundamental gauge boson masses. This thesis presents the results of the first search for the Higgs boson at the LHC, using 35 pb^-1 of proton-proton collision data with a centre of mass energy of 7 TeV collected by the ATLAS experiment throughout 2010. The search is performed in the H->WW channel, since the branching ratio for Higgs boson decays to W bosons is large for a wide range of Higgs boson masses. Two different search methods are presented: a straightforward cut-based method and a method using a matrix-element-based discriminant to provide additional separation between signal and background. The matrix element method is shown to give better expected sensitivity at all Higgs boson masses. Using these methods, a SM-like Higgs boson with a mass of 160 GeV with a production rate of 1.2 times the SM rate is excluded at 95% Confidence Level and limits are placed on the production rate of the SM Higgs boson in the range of masses from 120 < mH < 200 GeV. In addition, a measurement of the SM WW cross section is performed. It is essential to understand this channel since it is the major background to the H->WW search. SM WW production is also sensitive to new physics processes, which would enhance its cross section. The SM WW cross section is measured to be sigma_WW = 40^{+20}_{-16} (stat.) +/- 7(syst.) pb, which is consistent with the NLO SM expectation of 46 +/- 3 pb.