Abstract: 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 (??) in supersymmetric models that exhibit coannihilation between the ?? and the lightest neutralino (??01), 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 ?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 (??±1) or next-to-lightest neutralino (??02), and the ??01. The mass of the ?? is assumed to be the average of the ??±1 and ??01 masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the ??±1, ??02, and ?? production cross sections for ?m(??±1,??01)=50??GeV, resulting in a lower limit of 290 GeV on the mass of the ??±1, which is the most stringent to date and surpasses the bounds from the LEP experiments.

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