Search for direct top squark pair production in the 3-body decay mode with a final state containing one lepton, jets, and missing transverse momentum in $\sqrt{s}=13$TeV $pp$ collision data with the ATLAS detector

A search for direct pair production of top squarks, the supersymmetric partner of the top quark, is presented. The search focuses on final states with one isolated electron or muon, multiple jets, and large missing transverse momentum. The analysis is performed using the Large Hadron Collider Run 2 proton-proton dataset at a centre-of-mass energy of $\sqrt{s}=$13TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139fb$^{-1}$. One particular signal scenario is considered, characterised by the mass-splitting between the top squark and the lightest neutralino, where each top squark decays via a 3-body process to a $b$ quark, a $W$ boson, and a neutralino. No significant deviation from the predicted Standard Model background is observed, and limits at 95$\%$ confidence level on the supersymmetric benchmark model are set, excluding top squark masses up to 720GeV with neutralino masses up to 580GeV.