Study of the response of the electromagnetic calorimeter and research of a Higgs boson of 300 GeV/c2 in the channel qqH → qqWW → qqlνjj of the ATLAS experiment at LHC

The research of the Higgs boson implies a good uniformity of the response of the electromagnetic calorimeter of ATLAS for the study of its leptonic and photonic channels. In the first part of this thesis, I have developed a reliable monitoring tool for checking the quality of the data taking during these tests, shown that the procedure of production of the coefficients of optimal filtering comprises an artifact which influences the uniformity of the response in energy of a module and contributed to the installation of an algorithm of replacement of the missing coefficients. The second part relates to the study of the channel qqH → qqyWW → qqlvj j for a Higgs of 300 GeV/c32. The result reported in the technical design report did not allow to exploit this channel for a discovery because the distributions of the signal and backgrounds merged. Thanks to a new more global approach associated with a new cut on the mass of the Wj system and a relaxation of certain cuts, I have shown that this channel allows the discovery of the Higgs boson as well as the measurement of coupling H → WW. (author)