Observation d'une nouvelle particule dans la recherche du boson de Higgs se désintégrant en deux photons dans l'expérience ATLAS au LHC

This thesis presents a search for the Standard Model Higgs boson in the diphoton decay channel. It is based on the analysis of proton-proton collisions collected with the ATLAS detector at the Large Hadron Collider at CERN, in 2011 and 2012. The analyzed data correspond to 4.8 fb$^{-1}$ of integrated luminosity at a center-of-mass energy $\sqrt{s}=7$ TeV, and 5.9 fb$^{-1}$ at $\sqrt{s}=8$ TeV. In the diphoton invariant mass distribution, an excess of events is observed around 125 GeV. The probability to have a positive statistical fluctuation in that distribution equal or larger than the observed excess, anywhere in the explored mass range [$110 - 150$ GeV], is $1.6\times10^{-4}$. This probability corresponds to 3.6 standard deviations. Similar results have been obtained in the searches of the Higgs boson in other decay channels by the ATLAS collaboration, as well as by CMS. These results indicate the existence of a new particle, whose properties are compatible with those of the Standard Model Higgs boson.