Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum using 36.1fb$^{−1}$ of $\sqrt{13}$ TeV pp collision data with the ATLAS detector

The results of a search for the direct pair production of stops, 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 search uses data from $pp$ collisions delivered by the Large Hadron Collider in 2015 and 2016 at a center-of-mass energy of $\sqrt{13}$ TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36.1fb$^{-1}$. A wide range of signal scenarios with different mass splittings between the stop, the lightest neutralino and possible intermediate gauginos or higgsinos is considered, including cases where the $W$-bosons or the top-quarks are off-shell. The analysis also targets the production of dark matter in association with a pair of top-quarks using the same final state. 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 benchmark models, which reach up to a stop mass of 940 GeV. Stringent exclusion limits are also derived for all other considered stop decay scenarios, and upper limits are set on the visible cross-section for processes beyond the Standard Model.