Measurement of the $t\bar{t}t\bar{t}$ quark production in 13 TeV pp collisions with the ATLAS experiment

A measurement of four-top-quark production using 139 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider. Events are selected if they contain a single lepton (electron or muon) or an opposite-sign lepton pair, in association with multiple jets. The events are categorized according to the number of jets and how likely these are to contain b-hadrons. A multivariate technique is then used to discriminate between signal and background events. The measured four-top-quark production cross-section is $26^{+17}_{-15}$ fb, with a corresponding observed (expected) significance of 1.9 (1.0) standard deviations over the background-only hypothesis. The result is combined with the previous measurement performed by the ATLAS Collaboration in the multilepton final state. The combined four top-quark production cross-section is measured to be $24^{+7}_{-6}$ fb, with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over the background-only hypothesis. It is consistent within 2.0 standard deviations with the Standard Model prediction of 12.0 $\pm$ 2.4 fb.