Search for supersymmetry in events with at least one soft lepton, low jet multiplicity, and missing transverse momentum in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

A search for supersymmetry with a compressed mass spectrum using events with a high-momentum jet attributed to initial state radiation, high missing transverse momentum, and a low-momentum lepton is presented. In particular, scenarios of top squark pair production are investigated, where the mass difference to the lightest supersymmetric particle (LSP) is smaller than the mass of the W\,boson. The search is performed in a sample of proton-proton collisions recorded with the CMS detector at a centre-of-mass energy of $13~\mathrm{TeV}$ corresponding to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$. The results are consistent with the expectation from standard model processes and limits are set on the production cross section in the plane of the top squark vs. LSP masses. The signal models used for the interpretation of the results assume either four-body decays of the top squark, $\widetilde{t} \to \mathrm{b} f f' \mathrm{LSP}$, or decays via an intermediate chargino, $\widetilde{t} \to \mathrm{b} \widetilde{\chi}^+_1 \to \mathrm{b} f f' \mathrm{LSP}$. Assuming a $100\%$ branching ratio of the corresponding decay, top squark masses below $500~(540) ~\mathrm{GeV}$ for four-body (chargino-mediated) decay are excluded at $95\%$~confidence level for a mass difference to the LSP of about $30~\mathrm{GeV}$. These limits are currently the most stringent in the single lepton final state and are similar to those of analyses targeting other final states in this region of supersymmetry parameter space with 2016 data.