Recherche expérimentale de la brisure spontanée de symétrie électrofaible dans le canal $H \to \gamma \gamma$ et d'une solution au problème de hiérarchie dans ATLAS: Participation à la préparation de l'électronique du calorimètre électromagnétique

This thesis deals with the understanding of the spontaneous electroweak symmetry breaking mechanism in the ATLAS experiment at LHC collider, by studying two complementary thematics : the search for the Higgs boson in the H->gamma gamma channel, and a search for extra dimensions in the gluon sector. Tests of the electronic of the electromagnetic calorimeter allowed to validate various cards that were under the responsibility of the LPNHE. Using full simulation data of the detector allowed us to precisely compute mass resolution of the diphoton system. Due to recent theoretical improvements, signal and background have been studied at the next order of the perturbative development, which increases cross-sections. With regards to the jet background, a study has been done using discriminating variables in order to obtain, for a 80 % photons efficiency, a rejection factor of 7000. The discovery potential benefits from this change of cross-sections and increases by 52 % in comparison with the same analysis done at the leading order. In addition to this, a new analysis using a maximum likelihood method allowed us to increase by 40 % the discovery potential in comparison with our classical analysis. In conclusion, the Higgs boson of 120 GeV/c^2 can be now discovered in this channel with an integrated luminosity of 10 fb^{-1}. Furthermore, naturality problem of the Higgs boson mass can be solved by introducing extra dimensions in which gluons can propagate. We have shown that it was possible to discover extra-dimensions up to a compactication scale of 15 TeV. key words : Higgs, Photon, Extra-Dimensions, Kaluza-Klein, gluon, LHC, ATLAS, Electromagnetic Calorimeter, Liquid Argon, Electronics.