Single chargino production at linear colliders

We study the single chargino production $e^+ e^- \to \tilde \chi^{\pm} \mu^{\mp}$ at linear colliders which occurs through the $\l_{121}$ R-parity violating coupling constant. We focus on the final state containing 4 leptons and some missing energy. The largest background is \susyq and can be reduced using the initial beam polarization and some cuts based on the specific kinematics of the single chargino production. Assuming the highest allowed supersymmetric background, a center of mass energy of $\sqrt s=500GeV$ and a luminosity of ${\cal L}=500fb^{-1}$, the sensitivities on the $\l_{121}$ coupling constant obtained from the single chargino production study improve the low-energy experimental limit over a range of $\Delta m_{\tilde \nu} \approx 500GeV$ around the sneutrino resonance, and reach values of $\sim 10^{-4}$ at the $\tilde \nu$ pole. The single chargino production also allows to reconstruct the $\tilde \chi_1^{\pm}$, $\tilde \chi_2^{\pm}$ and $\tilde \nu$ masses. The initial state radiation plays a fundamental role in this study.