  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 2015 LHC $pp$ collision
data at a center-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. No
significant excess over the Standard Model background prediction is observed,
and exclusion limits on gluino and stop masses are set at 95% confidence level.
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 add an excluded stop mass region from 745 to 780 GeV for the direct
stop model with a massless lightest neutralino. The results are also
reinterpreted to set exclusion limits in a model of vector-like top quarks.
