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 high-momentum jet from initial state radiation, high missing energy, and a low-momentum lepton is presented. In particular, a scenario of top squark pair production is investigated, where the mass difference to the lightest supersymmetric particle (LSP) is smaller than the mass of the $\mathrm{W}~\mathrm{boson}$. In this case the top squarks could undergo four-body decays to $\mathrm{b} f f' + \mathrm{LSP}$. 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 $12.9~\mathrm{fb}^{-1}$. 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. Assuming a $100\%$ branching ratio of the four-body decay, top squark masses below $330~\mathrm{GeV}$ are excluded at $95\%$ confidence level for a mass difference to the LSP of about $25~\mathrm{GeV}$. The sensitivity of this search is similar to that of analyses targeting other final states in this region of supersymmetry parameter space with 2016 data.