Search for new phenomena with top-quark pairs in final states with one lepton, jets and missing transverse momentum using 140 $\mathrm{fb}^{-1}$ of data at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for new phenomena with top quark pairs in final states with one isolated lepton, either an electron or a muon, jets, and large missing transverse momentum is performed. The search is performed using the Large Hadron Collider proton--proton collision dataset at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 140 fb$^{-1}$. Event classifiers based on neural networks are optimised to search for directly produced pairs of supersymmetric partners of the top quark (stop), as well as to search for spin-0 mediators produced in association with a pair of top quarks and decaying into dark-matter particles. A novel analysis approach is developed to achieve high sensitivity over a wide region of the parameter space of the tested signal models. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop–neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Models with neutralinos from the direct stop decay with masses up to 600 GeV are excluded, while at small neutralino masses models are excluded for stop masses up to 1090 GeV. Scalar (pseudoscalar) dark matter mediator masses as large as 250 (300) GeV are excluded when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one. At lower mediator masses, models with production cross sections as small as 0.22 (0.26) times the nominal predictions are excluded.