Search for supersymmetry with a compressed mass spectrum in events with a soft tau lepton, a highly energetic jet, and large missing transverse momentum in proton-proton collisions at sqrt(s) = 13 TeV

The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying τ 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 τ leptons ( ̃τ) in supersymmetric models that exhibit coannihilation between the ̃τ and the lightest neutralino ( ̃χ01), 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 (Δm) between the chargino ( ̃χ±1) or next-to-lightest neutralino ( ̃χ02), and the ̃χ01. The mass of the ̃τ is assumed to be the average of the ̃χ±1 and ̃χ01 masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the ̃χ±1, ̃χ02, and ̃τ production cross sections for Δm( ̃χ±1, ̃χ01)=50 GeV, resulting in a lower limit of 290 GeV on the mass of the ̃χ±1, which is the most stringent to date and surpasses the bounds from the LEP experiments.