Abstract: 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 WWW 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

Results are presented from a search for the electroweak production of supersymmetric particles in pp collisions in final states with two tau leptons. The data sample corresponds to an integrated luminosity between 18.1 inverse femtobarns and 19.6 inverse femtobarns at sqrt(s) = 8 TeV, collected by the CMS experiment at the LHC. The observed event yields in the signal regions are consistent with the expected standard model backgrounds. The results are interpreted using simplified models describing the pair production and decays of charginos or tau sleptons. For chargino pair production, assuming that the third-generation sleptons are the lightest sleptons and that their masses lie midway between that of the chargino and the lightest neutralino, an excluded region in the plane of the neutralino mass vs. chargino mass is obtained. Chargino masses below 420 GeV are excluded at a 95% confidence level in the limit of a massless neutralino, and for neutralino masses up to 100 GeV, chargino masses up to 325 GeV are excluded at 95% confidence level. Constraints are also placed on the cross section for pair production of tau sleptons as a function of mass, assuming a massless neutralino.