Search for New Physics with the Study of Multi-lepton Final States with the ATLAS Experiment at the LHC at CERN

The search for diboson resonances is an essential probe for understanding the source of elec- troweak symmetry breaking (EWSB). In the Standard Model (SM), the strength of the coupling of the Higgs scalar to vector boson pairs is well predicted, but it is well known that the low mass of the Higgs boson leads to hierarchy and naturalness problems suggesting that the SM is only an effective theory at low masses. As a result, there is need for New Physics to be introduced for higher masses, focusing on multi-lepton final states which provide us with clear signal sig- natures. The present thesis targets on the search for resonances predicted by two distinct signal hypothesis: a spin-1 W ′ from the Parametrization of heavy vector triplet (HVT) Lagrangians and a spin-0 charged higgs H ± , predicted by Georgi-Machacek (GM) model. More specifi- cally, we study the resonant WZ diboson production using 139 f b −1 data collected at 13 TeV center of mass energy by the ATLAS detector at the LHC, where only the leptonic final states (e or μ) are considered for the W and Z boson decays. Limits on the production cross-section times branching ratio are obtained as a function of the resonance mass for resonances arising from two different models, Heavy Vector Triplet and Georgi-Machacek model. Two different production modes are considered, the vector boson fusion and the Drell-Yan process (quark- anti quark fusion), on which independent limits are set. Although the nominal discriminating variable for the limits setting procedure is the invariant mass of the WZ system, different op- tions are examined too and limits are obtained using alternative variables which in some cases result in stricter limits.