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 $\sqrt{s} =$ 13 TeV

Abstract

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 ($\tildeτ$) in supersymmetric models that exhibit coannihilation between the $\tildeτ$ and the lightest neutralino ($\tildeχ^0_1$), 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 ($Δm$) between the chargino ($\tildeχ^\pm_1$) or next-to-lightest neutralino ($\tildeχ^0_2$), and the $\tildeχ^0_1$. The mass of the $\tildeτ$ is assumed to be the average of the $\tildeχ^\pm_1$ and $\tildeχ^0_1$ masses. The data are consistent with standard model background predictions. Upper limits at 95% confidence level are set on the sum of the $\tildeχ^\pm_1$, $\tildeχ^0_2$, and $\tildeτ$ production cross sections for $Δm(\tildeχ^\pm_1, \tildeχ^0_1) =$ 50 GeV, resulting in a lower limit of 290 GeV on the mass of the $\tildeχ^\pm_1$, which is the most stringent to date and surpasses the bounds from the LEP experiments.