Search for new phenomena with top quark pairs in final states with one lepton, jets, and missing transverse momentum in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are employed to search for two- and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediator models, where the mediator is produced in association with a pair of top quarks and decays into a pair of dark matter particles. The search is performed using the Large Hadron Collider proton-proton dataset at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb$^{−1}$. No significant excess from the Standard Model background is observed, and limits at 95% confidence level are set in the stop–neutralino and in the mediator–dark matter particle mass planes. Stops are excluded up to 1200 GeV in the two-body decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop–neutralino mass difference of 60 GeV. Scalar and pseudo-scalar dark matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to standard model and dark matter particles are both equal to one and when the mass of dark-matter is 1 GeV.