The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying $��$ lepton, one energetic jet from initial-state radiation, and large transverse momentum imbalance is presented. These event signatures are consistent with direct or indirect production of scalar $��$ leptons ($\tilde��$) in supersymmetric models that exhibit coannihilation between the $\tilde��$ and the lightest neutralino ($\tilde��^0_1$), and that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of 77.2 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} =$ 13 TeV collected with the CMS detector at the LHC in 2016 and 2017. The results are interpreted in a supersymmetric scenario with a small mass difference ($��m$) between the chargino ($\tilde��^\pm_1$) or next-to-lightest neutralino ($\tilde��^0_2$), and the $\tilde��^0_1$. The mass of the $\tilde��$ is assumed to be the average of the $\tilde��^\pm_1$ and $\tilde��^0_1$ masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the $\tilde��^\pm_1$, $\tilde��^0_2$, and $\tilde��$ production cross sections for $��m(\tilde��^\pm_1, \tilde��^0_1) =$ 50 GeV, resulting in a lower limit of 290 GeV on the mass of the $\tilde��^\pm_1$, which is the most stringent to date and surpasses the bounds from the LEP experiments.

Replaced with the published version. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/SUS-19-002 (CMS Public Pages)