Evidence for $t\overline{t}t\overline{t}$ production in the multilepton final state in proton–proton collisions at $\sqrt{s}=13$ $\text{TeV}$ 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 $13\phantom{\rule{3.33333pt}{0ex}}\text{TeV}$ 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 ${24}_{-6}^{+7}$ 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.