Photon performance studies with Radiative Z decays, and the search for the Higgs boson in the H → γγ and H → Zγ channels with the ATLAS detector of the LHC

In this thesis my personal contributions to the ATLAS experiment are presented, consisting of detector oriented studies and physics analyses concerning photons in the context of the search for the Higgs boson. The first part of this thesis contains detector performance analyses on the electromagnetic calorimeter (EMCAL) high-voltage system. The effect of the resistors in the electrodes of the EMCAL on the energy measurement is investigated and found to be small in most of the cases. Furthermore, photon reconstruction performance studies are presented, where a data-driven validation to the standard calibration is performed by extracting the photon energy scales from a sample of Z radiative decays. The second part of this document concerns the physics analyses, such as the searches for the Higgs Boson in the γγ and Zγ decay channels. My main contribution to these analysis consists of an analytical resolution model for the signal, built to satisfy the need for an interpolation of the invariant mass probability density function to perform the search. The di-photon decay channel results uses 4.8 fb−1 of data recorded at a centre-ofmass energy of √s = 7 TeV and 20.7 fb−1 at √s = 8 TeV. An excess of events is observed around a mass of 126.5 GeV with a local significance of 4.5 standard deviations. These results, combined with those obtained in the in the H → ZZ and H → WW channel confirm the discovery of a new boson with a mass of 126.0 ± 0.4(stat)±0.4(sys) GeV, consistent with the long searched-for Higgs boson. In the H → Zγ channel, the search is performed using 4.8 fb−1 at f √s = 7 TeV and 20.7 fb−1 at √s = 8 TeV. No significant deviations from the SM background expectations are observed and upper limits on the cross sections times branching ratio are set. For a mass of 125 GeV, the expected and observed limits are 13.5 and 18.2 times the SM, respectively.