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 $\sqrt{s}=$13 TeV and collected by the ATLAS detector during the 2015 and 2016 data-taking. Events containing leptons with same electric charge allow to reject a substantial fraction of the background from the Standard Model processes. New physics is searched both indirectly and directly through two processes: the production of a Higgs boson in association with a pair of top quarks ($t\bar{t}H$) and the production of new doubly charged Higgs bosons ($H^{\pm\pm}$). The measurement of the $t\bar{t}H$ 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 $\mu _{t\bar{t}H}=1$. Regarding direct searches, no evidence for new physics is found in the search for $H^{\pm\pm}$ boson production: lower limits on its mass are set and vary from 770 GeV to 870 GeV at 95% confidence level.