## Characterization of the Background due to Lepton Scattering to the $\chi^{\pm} \rightarrow \chi^{0} \, \pi^{\pm}$ Disappearing Track Signature with the ATLAS Detector
Supersymmetry models in which the lightest particle is pure-wino or pure-higgsino have chargino next-to-lightest-particles (NLSP) nearly mass-degenerate with the lightest neutralino, resulting in a lifetime long enough to produce hits in the Pixel Detector but not long enough to produce a full track; this is called the disappearing track signature". Electrons and muons which traverse the Pixel Detector and Coulomb scatter at a steep angle and therefore fail to be reconstructed as electrons or muons are a significant background to this signature; this is the lepton scatter background". This lepton scatter background was characterized using $36 \, \mathrm{fb}^{-1}$ of integrated luminosity of proton-proton collisions at $\sqrt{s} = 13 \, \mathrm{TeV}$ center-of-mass energy in the ATLAS Detector. Electrons were found to dominate at lower transverse momentum whereas at above about $100 \, \mathrm{GeV}$ the contributions are similar. Comparison with a representative pure-wino Monte Carlo model suggests a minimum threshold of $p_{T} > 100 \, \mathrm{GeV}$ should be used in the full analysis.