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

The results of a search for the stop, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the LHC pp collision data collected in 2015 and 2016 at a center-of-mass energy of $\sqrt{s}$= 13 TeV recorded by the ATLAS detector corresponding to an integrated luminosity of 13.2 fb$^{−1}$. The stop decay scenarios considered are those to a top quark and the lightest neutralino as well as to a bottom quark and the lightest chargino, followed by the chargino decays to the lightest neutralino and a $W$ boson. A range of scenarios with different mass splittings between the stop, the lightest chargino and the lightest neutralino are considered. The analysis also targets the production of dark matter in association with a pair of top quarks using the same final state. The largest deviation from the Standard Model prediction is found in one of the seven signal selections, where 35 events are observed with 17$\pm$2 background events expected, corresponding to a local significance of 3.3 standard deviations. A stop with a mass of 830 GeV decaying directly to a top quark and a massless neutralino is excluded at 95% confidence level. 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. For the dark matter search, limits are obtained for the common coupling value of $g$ = 3.5 in a plane of dark matter particle mass and a scalar or pseudo-scalar mediator mass. The maximal coupling of $g$ = 3.5 is excluded at 95% confidence level for a scalar (pseudo-scalar) mediator mass up to 320 (350) GeV assuming a 1 GeV dark matter particle mass.