Search for long-lived charginos based on a disappearing-track signature in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Nearly degenerate electroweak gaugino mass eigenstates appear in many supersymmetric scenarios. They can give rise to long-lived charginos, nearly degenerate in mass with the stable neutralino, yielding the distinctive experimental signature of a track disappearing within the detector. This paper presents a search for direct electroweak gaugino or gluino pair production with the chargino nearly mass-degenerate with a stable neutralino. It is based on the integrated luminosity of 36.1 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 13$ TeV by the ATLAS experiment at the LHC. The final states of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95\% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 430 GeV are excluded. For strong production, assuming 0.2 ns of chargino lifetime, gluino masses up to 1.6 TeV are excluded assuming a chargino mass of 430 GeV.