Search for Supersymmetry with a Compressed Mass Spectrum in Events with a Soft 
τ
 Lepton, a Highly Energetic Jet, and Large Missing Transverse Momentum in Proton-Proton Collisions at 
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 (τ̃ ) in supersymmetric models that exhibit coannihilation between the τ̃ and the lightest neutralino (χ̃1), 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 (χ̃± 1 ) or next-to-lightest neutralino (χ̃ 0 2), and the χ̃ 0 1. The mass of the τ̃ is assumed to be the average of the χ̃± 1 and χ̃ 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 χ̃± 1 , χ̃ 0 2, and τ̃ production cross sections for ∆m(χ̃ ± 1 , χ̃ 0 1) = 50 GeV, resulting in a lower limit of 290 GeV on the mass of the χ̃± 1 , which is the most stringent to date and surpasses the bounds from the LEP experiments. Submitted to Physical Review Letters c © 2019 CERN for the benefit of the CMS Collaboration. CC-BY-4.0 license ∗See Appendix A for the list of collaboration members ar X iv :1 91 0. 01 18 5v 1 [ he pex ] 2 O ct 2 01 9