Sensitivity to sterile neutrino mixings and the discovery channel at a neutrino factory

Sensitivity of a neutrino factory to various mixing angles in a scheme with one sterile neutrino is studied using $\nu_e\to\nu_\mu$, $\nu_\mu\to\nu_\mu$, $\nu_e\to\nu_\tau$ and $\nu_\mu\to\nu_\tau$. While the "discovery-channel" $\nu_\mu\to\nu_\tau$ is neither useful in the standard three flavor scheme nor very powerful in the sensitivity study of sterile neutrino mixings, this channel is important to check unitarity and to probe the new CP phase in the scheme beyond the standard neutrino mixing framework.