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

A search for the stop, the supersymmetric partner of the top quark, is conducted in final states with one isolated electron or muon, jets, and missing transverse momentum using the 2015 LHC $pp$ collision data at a centre-of-mass energy of $sqrt{s}=13$ TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 3.2 fb${}^{-1}$. The analysis targets two types of signal models: gluino-mediated pair production of stops with a nearly mass degenerate stop and neutralino; and direct pair production of stops, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum, $t\bar{t} + E_\text{T}^\text{miss}$. No significant excess over the Standard Model prediction is observed, and exclusion limits on gluino and stop masses are set at 95% CL. The results extend the LHC Run 1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low stop mass region, and for the direct stop model add an excluded stop mass region from 745 to 780 GeV for a massless lightest neutralino.