Studies of Four-Top-Quark Production in Proton-Proton Collisions at the LHC With The Same-Sign Dilepton and Multilepton Final States

Abstract

This dissertation presents the measurements and searches of four-top-quark (tttt) production with same-sign dilepton and multilepton final states in proton-proton collision data collected using the ATLAS detector at the LHC. Four-top-quark production, a rare process in the Standard Model (SM) with a cross section around 12 fb at a center-of-mass energy of 13 TeV, is one of the heaviest final states produced at the LHC, and it is naturally sensitive to beyond SM (BSM) physics. An early analysis by the ATLAS experiment resulted in an evidence for SM tttt production with a significance of 4.3 standard deviations with respect to the background-only hypothesis, but the measured tttt cross section is twice the SM prediction. This dissertation focuses on three major topics on the tttt production: the observation of the SM tttt production, a search of BSM tttt production (ttH/A->tttt) predicted in the type-II Two-Higgs-Doublet Model (2HDM), and a reinterpretation analysis of BSM tttt search in a 2HDM+a combination analysis.