Recherche de manifestations de dimensions supplémentaires dans le canal diphoton avec l'expérience ATLAS au LHC

This thesis summarizes a search for manifestations of Large Extra Dimensions (LED) using 4.91~$fb^{-1}$ of data collected in 2011 by the Atlas detector at the LHC collider at CERN. In 2011, the LHC has provided proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 7$~TeV. LED can potentially solve the so-called hierarchy problem, i.e. large apparent difference between two fundamental scales of the Standard Model (SM), the electroweak and the Planck scales. In the context of the ADD model (named after the authors N. Arkani-Hamed, S. Dimopoulos and G. Dvali) of LED, the effects of quantum gravity become much stronger than in the SM; possibly large enough to be observed at the LHC. There are two possibilities of graviton production in proton-proton collisions: direct graviton production and virtual graviton exchange. In this thesis, we present a search for the manifestation of extra dimensions via the effect of virtual graviton exchange on the di-photon final state. The di-photon invariant mass spectrum is studied and found to be in good agreement with SM background expectation. We set limits on the fundamental Planck scale of the ADD model using two different methods: a counting experiment and an analysis of the shape of the di-photon mass spectrum. The counting experiment yields limits between 2.62 and 3.92~TeV at 95\% C.L., depending on the number of extra dimensions and the theoretical formalism used. The shape analysis yields slightly more stringent limits: the lower limits on the fundamental Planck scale improve by a factor of 1.04.