Observation of the Higgs particle in γγ events and search for the Higgs particle in Zγ events at ATLAS

This thesis focuses on the searches for the Higgs boson in events with photons in the √ final states, using the full proton-proton collision data collected by ATLAS at s = 7 and 8 TeV in 2011 and 2012. Higgs boson decays to photon pairs or to a photon and a Z boson decaying to di-electrons or di-muons are investigated. The event selection, the main backgrounds, the signal properties, and the statistical discrimination between the signal and background in data and the interpretation of the results in terms of a Standard Model Higgs boson are discussed. In the H → γγ channel a clear excess over the background is seen at a mass of m H = 126.8 ± 0.2(stat) ± 0.7(syst) GeV, with a local significance of 7.4σ. In the rare decay channel H → Zγ no evidence of excess over the background is observed in the mass range 120-150 GeV, and, for a Higgs boson mass near the one obtained from the combined mass measurement in the γγ and 4-lepton final states, m H = 125.5GeV , an upper limit of 11 times the SM prediction, at 95% confidence level, is set on the production cross section times the H → Zγ cross section. One of the most important ingredient for these measurements is the efficient reconstruction and identification of photons, and a precise knowledge of the trigger and identification performance. A significant part of the document is thus devoted to the photon performance optimisation and measurement activities that I carried on in the past three years.