Measurement of $t\bar{t}t\bar{t}$ Production Cross Section and Search for New Physics using the Multilepton Final State in Proton-Proton Collisions at $\sqrt{s}$= 13 TeV with the ATLAS Detector

Four top-quark production, a rare process in the Standard Model (SM) with a cross-section around 12 fb, is one of the heaviest final states produced at the LHC. It is therefore naturally sensitive to physics beyond the Standard Model (BSM). An excess is observed in the data with the measured rate being twice of the expectation in an earlier analysis. A follow-up analysis aims to constrain non-resonant BSM physics including Yukawa coupling, Effective Field Theory, Higgs oblique parameter and three top-quark production as well as reoptimization of the SM four-tops measurement. The data analyzed correspond to an integrated luminosity of 139 fb$^{-1}$ collected using the ATLAS detector apparatus. In this talk, the four top-quark decay final states containing either a pair of same-sign leptons or multiple leptons are considered. Sensitivity improvements with novel techniques on multivariate analysis and improved modeling of the jet multiplicity spectrum using a parameterized data-driven model will be discussed.