Search for charginos nearly mass-degenerate with the lightest neutralino based on a disappearing-track signature in $pp$ collisions at $\sqrt{s}=8~\mathrm{TeV}$ with the ATLAS detector

A search is presented for direct chargino production based on a disappearing-track signature using 20.3 $\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s} = 8~\mathrm{TeV}$ collected with the ATLAS experiment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the lightest chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is long enough to be detected in the tracking detectors by identifying decays that result in tracks with no associated hits in the outer region of the tracking system. A number of models with supersymmetry also predict charginos with a significant lifetime. This analysis reaches a sensitivity for charginos with a lifetime between $1~\mathrm{ns}$ and $10~\mathrm{ns}$ and masses up to about $500~\mathrm{GeV}$ and for the first time significantly extends the reach of the LEP experiments for charginos with lifetimes $> 0.1~\mathrm{ns}$. No significant excess beyond the background expectation is observed for candidate tracks with large transverse momentum, and constraints on chargino properties are obtained. In the AMSB scenarios, a chargino mass below $270~\mathrm{GeV}$ is excluded at 95% confidence level.