Search for top squarks decaying via four-body or chargino-mediated modes in the single-lepton final state at $\sqrt{s} = 13~\mathrm{TeV}$

A search for top squark pair production in a compressed scenario, where the mass difference between the top squark and the lightest supersymmetric particle (LSP) is smaller than the mass of the W boson, is presented. The dataset is comprised of proton-proton collisions, recorded by the CMS experiment at a centre-of-mass energy of $13~\mathrm{TeV}$ and corresponding to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$. In this search two decay modes of the top squark are considered: a four-body decay into a b quark, two additional fermions, and an LSP, and a decay via an intermediate chargino. Events are selected using the presence of a high-momentum jet, significant missing transverse energy and a low transverse momentum electron or muon. Two analysis techniques are used: a sequential selection based on the most discriminating observables, and a multivariate technique targeting the four-body decays. No evidence for the production of top squarks is found and mass limits at $95\%$ confidence level are set that reach up to $560~\mathrm{GeV}$, depending on the m($\widetilde{t}_1$)-m($\widetilde{\chi}^0_1$) mass difference and the decay mode.