ATLAS Cross Section Measurement of $t\bar{t}t\bar{t}$ in the Single-Lepton and Opposite Sign Dilepton Channel

The methodology and results are presented for the search for four-top production at the ATLAS detector. An integrated luminosity of 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV was analyzed to measure the cross section of the four-top production, specifically for decays containing 1 lepton or 2 oppositely-charged leptons. For these largely hadronic decay channels, signal regions were defined by high jet and b-tagged jet multiplicity. Within these regions, a multivariate discriminant provided further isolation of the four-top-quark signal from backgrounds, in particular the dominant background resulting production of top-quark pair plus jets. A reweighting technique was applied to ensure that the backgrounds were accurately modelled to avoid biasing the multivariate discriminant in regions of high jet multiplicity. The analysis measured a cross section value of 26 (+17/-15) fb, or an observed (expected) discrepancy of 1.9 (1.0) standard deviations from the background-only hypothesis.