Search for ttbar resonances in final states with exactly one or two leptons using 140 fb${}^{-1}$ of pp collision data at $\sqrt{s}=13$ TeV with the ATLAS experiment

A search for heavy spin-1 and spin-2 resonances decaying into a top-antitop-quark pair has been performed with 140 fb${}^{-1}$ of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Final states with either exactly one electron or muon, or exactly two leptons ($ee$, $\mu \mu$ or $e\mu$), large missing transverse momentum, and two jets, at least one of which must be identified as likely containing a b-hadron decay, are considered. The search targets resonances with both narrow and broad widths relative to the detector resolution, and with masses in the range of 0.4-5.0 TeV. No significant deviation from the Standard Model prediction is observed. Exclusion limits are set on the production cross-section times branching ratio for hypothetical $Z^{\mathrm{\prime }}$ bosons, Kaluza-Klein gravitons, and Kaluza-Klein gluons that decay into top-quark pairs.