Evidence for $$t\bar{t}t\bar{t}$$tt¯tt¯ production in the multilepton final state in proton–proton collisions at $$\sqrt{s}=13$$s=13 $$\text {TeV}$$TeV with the ATLAS detector

A search is presented for four-top-quark production using an integrated luminosity of 139 fb $$^{-1}$$ of proton–proton collision data at a centre-of-mass energy of $$13~\text {TeV}$$ collected by the ATLAS detector at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be $$24^{+7}_{-6}$$  fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.