Measurement of $WWW$ production and search for supersymmetric particles in multi-lepton final states in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC

The results presented in this thesis use the full dataset collected in 2012 by the ATLAS experiment from the LHC proton-proton collisions at $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The first part is devoted to the search for the $W^{\pm}W^{\pm}W^{\mp}$ production in the decay channel where each $W$ boson decays leptonically (excluding leptonic $\tau$ decays). Upper limits are placed on the production cross-section in the fiducial phase space. The results are also interpreted in terms of the total cross-section when combined together with the semi-leptonic decay channel. In addition, limits on anomalous quartic gauge boson couplings in an effective field theory approach are extracted. The second part presents a search for supersymmetry in events with multiple jets, two leptons of identical charges or three leptons (electrons or muons). The absence of any excess in the signal regions with respect to the Standard Model expectations is interpreted in terms of limits in the parameter space of 14 supersymmetric models and one mUED model.