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

A search is presented for direct chargino production based on a disappearing-track sig-nature using 20.3 fb1 of pp collisions atps = 8 TeV collected with the ATLAS experi-ment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the light-est chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is longenough to be detected in the tracking detectors by identifying decays that result in trackswith no associated hits in the outer region of the tracking system. A number of models withsupersymmetry also predict charginos with a significant lifetime. This analysis reaches asensitivity for charginos with a lifetime between 1 ns and 10 ns and masses up to about500 GeV and for the first time significantly extends the reach of the LEP experiments forcharginos with lifetimes > 0:1 ns. No significant excess beyond the background expecta-tion is observed for candidate tracks with large transverse momentum, and constraints onchargino properties are obtained. In the AMSB scenarios, a chargino mass below 270 GeVis excluded at 95% confidence level