Recherche de Nouvelle Physique dans les événements à quatre quarks top avec le détecteur ATLAS auprès du LHC

Despite its numerous success, like the recent discovery of a boson compatible with the BEH boson, the Standard Model shows some deficiencies. Those lead to a search for New Physics, which may take, among others, the form of extra space dimensions. In this thesis, four-top-quarks events arising from a such model, the 2UED/RPP, are searched for. The 2UED/RPP has two universal extra space dimensions compactified under the Real Projective Plane geometry. This search uses a signature with at least two same-sign leptons (electrons and/or muons). Two successive analyses are presented. The first one uses the first 14.3 fb$^{-1}$ recorded during 2012 proton-proton collisions occurring with a collision center-of-mass referential energy of 8 TeV. In the case where both extra dimensions have the same compactification radius and where the branching ratio of $A^{(1\,;\,1)}$ in $t\bar{t}$ is 100%, this analysis puts a lower observed (expected) limit on $m_\mathrm{KK}$ (main model parameter) at 0.90 TeV (0.92 TeV) with a confidence level of 95%. The second analysis extend the first one to the full 20.3 fb$^{-1}$ dataset while refining the procedures in use. An excess of data with a significance of less than 2.5 standard deviations is observed. None of the cross-checks done shows any default which could explain this excess. The new limit is 0.96 TeV (1.05 TeV). This second analysis also allows to put limits on the model's parameters in other configurations. Finally, this document also presents the author's contribution to the first stages of the LaserII development. The LaserII is the new LASER calibration system of ATLAS's Tile Calorimeter.