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 offshell. 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.The search presented is based on 16 dedicated analyses that target the various scenarios mentioned above. Each of these analyses corresponds to a set of event selection criteria, referred to as a signal region (SR), and is optimised to target one or more signal scenarios. Two different analysis techniques are employed in the definition of the SRs, which are referred to as 'cut-and-count' and 'shape-fit'. The former is based on counting events in a single region of phase space, and is employed in the 16 analyses. The latter is used in some SRs in addition to the 'cut-and-count' technique and employs SRs split into multiple bins in a specific discriminating kinematic variable, that can cover a range that is larger than the 'cut-and-count' SR. By utilising different signal-to-background ratios in the various bins, the search sensitivity is enhanced in challenging scenarios where it is particularly difficult to separate signal from background.The main background processes after the signal selections include tt, single-top Wt, tt + Z(→ νν), and W+jets. Each of those SM processes are estimated by building dedicated control regions (CRs) enhanced in each of the processes, making the analysis more robust against potential mis-modelling effects in simulated events and reducing the uncertainties in the background estimates. The backgrounds are then simultaneously normalised in data using a likelihood fit for each SR with its associated CRs. The background modelling as predicted by the fits is tested in a series of validation regions (VRs).
ATLAS detector and data collectionThe ATLAS detector [82] is a multipurpose particle physics detector with nearly 4π co...