Search for Type-III SeeSaw heavy leptons in leptonic final states using proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This thesis analyzes multi-leptonic final states to perform a search for the pair production of heavy leptons as predicted by the Type-III SeeSaw mechanism. The analysis uses proton-proton collision data at a centre-of-mass energy of 13 \units{TeV}, corresponding to 139 \invfb\ of integrated luminosity recorded by the ATLAS detector during the full Run 2 period of the Large Hadron Collider (2015-2018). The search is focused on final states with three and four light leptons (electrons and muons) and large energy produced by the collision coming from all the possible decays of the heavy leptons via intermediate electroweak bosons. As no significant excesses with respect to the Standard Model predictions are observed, upper and lower limits on the production cross-section and on the heavy lepton masses are set. The observed lower mass limit is 870 \gev\ at 95\% of confidence level considering both three- and four-lepton channels. In addition, this result is combined for the first time with the ones already published by the ATLAS Collaboration in the two leptons plus two jets final state. In this case, the observed lower mass limit combining two, three and four lepton channels is 910 \gev\ at the 95\% of confidence level.