Study of multi-lepton final states with the ATLAS experiment: from Standard Model to Beyond Standard Model Higgs.

This thesis exploits the features of final states containing light leptons with same electric charge, produced in LHC proton-proton collisions at 13 TeV and collected by the ATLAS detector during the 2015 and 2016 data-taking. Events containing leptons with same electric charge allow to reject a substantial fraction of the background from the Standard Model processes. New physics is searched both indirectly and directly through two processes: the production of a Higgs boson in association with a pair of top quarks (ttH) and the production of new doubly charged Higgs bosons (H++). The measurement of the ttH signal strength (mu) into leptonic final states provides mu = 2.5 (+1.3 - 1.1), compatible at 95% confidence level with the Standard Model prediction mu(ttH) = 1. Regarding direct searches, no evidence for new physics is found in the search for H++ boson production: lower limits on its mass are set and vary from 770 GeV to 870 GeV at 95% confidence level.