The Search for the Standard Model Higgs Boson in $H \to \gamma\gamma$ Decays with the ATLAS Detector in 4.9 fb$^{-1}$ of 2011 Data at $\sqrt{s}$ = 7 TeV

The ATLAS detector at the LHC collider located at CERN is designed to unveil physics of the smallest building blocks of Nature in energy domains previously unseen. Integrated in the Standard Model of elementary particle physics is a mechanism for breaking the electroweak symmetry, which performs the non-trivial task of assigning mass to the elementary constituents, commonly called the Brout- Englert-Higgs (BEH) mechanism. In order to examine whether the field introduced in this mechanism exists or not, searches for the manifestation of the field, often called the Higgs boson, are performed. This thesis addresses the search for the Standard Model Higgs boson through its decay to two photons, one of the most promising ways of discovering the particle, should it exist. Diphoton candidate events in 4.9 fb$^{-1}$ of 2011 data at a center-of-mass energy of $\sqrt{s}$ = 7 TeV are selected and sorted into nine different categories, based on mass resolution and signal-to-background ratio. One of the criteria for selecting these Higgs boson candidate events is lower thresholds on the transverse momenta of the two photons. These cuts had not been re-evaluated since the 1990’s; in this thesis, cuts which vary linearly with the Higgs boson invariant mass are studied. The high production rate of jets at the LHC, in combination with the high jet rejection factor needed to be able to resolve a signal in this decay channel, make it impossible to produce full Monte Carlo background simulation samples with sufficient statistics. Therefore, the normalization and shape of the background is taken solely from fits to the nine categories of data. Properties of several background parametrizations were investigated in detail, and the choice of which ones to use for the different categories was made “blindly” in Spring 2012, i.e. before looking at the 2012 data. As an alternative to the statistical procedure of ATLAS for evaluating the support or lack of evidence for a Higgs boson signal in data, a method in which each event is given a weight based on the expected sensitivity of the category it belongs to is implemented. The resulting weighted invariant mass spectrum was used to evaluate the significance of the excess seen in 2011 data. At the mass where the maximum excess is seen, m$_{H}$ = 126.5 GeV, the significance expected for the SM Higgs boson is 1.4 σ, while the observed significance is 1.9 σ. These results are roughly compatible with published ATLAS results for $H \to \gamma\gamma$, which were used for the combined result in the article claiming the discovery of a ‘Higgs boson’-like particle in July 2012