Abstract
The results of a search for the direct pair production of top squarks, the supersymmetric partner
of the top quark, in final states with one isolated electron or muon, several energetic jets, and
missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where
the mediator decays into a pair of dark-matter particles and is produced in association with a pair
of top quarks. The search uses data from proton–proton collisions delivered by
the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of √s = 13 TeV
and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb−1. A wide
range of signal scenarios with different mass-splittings between the top squark, the lightest
neutralino and possible intermediate supersymmetric particles are considered, including cases where
the W bosons or the top quarks produced in the decay chain are off- shell. No significant excess
over the Standard Model prediction is observed. The null results are used to set exclusion limits
at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks
decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits
are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator
models, upper limits are set on the visible
cross-section.