Search for top squarks decaying via the four-body mode in single-lepton final states from Run 2 of the LHC

Results are presented from a search for the pair production of the lightest supersymmetric partner of the top quark ($\mathrm{\widetilde{t}}_{1}$). The search targets the four-body decay of the $\mathrm{\widetilde{t}}_{1}$, which is allowed when the mass difference between the top squark and the lightest supersymmetric particle is smaller than the mass of the $\mathrm{W}$ boson. This decay mode consists of a bottom quark, two other fermions, and a $\widetilde{\chi}^{0}_{1}$, often considered to be the lightest supersymmetric particle. The data correspond to an integrated luminosity of $138~\mathrm{fb}^{-1}$ of proton-proton collisions at a center-of-mass energy of $13~\mathrm{TeV}$, collected with the CMS detector. The signature of the selected events is defined by a high-momentum jet, significant missing transverse momentum, and a low transverse momentum electron or muon. The selection of the signal is based on a multivariate approach that is adapted to the $m(\mathrm{\widetilde{t}}_{1}) - m(\widetilde{\chi}^{0}_{1})$ mass difference. Leading background processes are determined from data. No significant excess in data is observed above the expectation from standard model processes. The results of this search exclude top squark masses up to $700~\mathrm{GeV}$, depending on the $m(\mathrm{\widetilde{t}}_{1}) - m(\widetilde{\chi}^{0}_{1})$ mass difference.