Optimization of a search for top squark decays to a b-quark and a chargino in final states with one lepton in $pp$ collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector

We present the results of an optimization study for a top squark search signal region. The scenario studied is the decay to a b-quark and a chargino, with compressed chargino and neutralino mass splitting. This compressed scenario leads to an off-shell W-boson, which produces a low-energy final state lepton. For high top squark masses, the final state has a high-energy b-jet and large missing transverse energy. Using this signature, we optimize the signal region for discrimination against the Standard Model background across a range of signal mass points.