Probing nonunitary mixing and CP violation at a neutrino factory

A low-energy nonunitary leptonic mixing matrix is a generic feature of many extensions of the standard model. In such a case, the task of future precision neutrino oscillation experiments is more ambitious than measuring the three mixing angles and the leptonic (Dirac) $CP$ phase, i.e., the accessible parameters of a unitary leptonic mixing matrix. A nonunitary mixing matrix has 13 parameters that affect neutrino oscillations, out of which four are $CP$ violating. In the scheme of minimal unitarity violation we analyze the potential of a neutrino factory for determining or constraining the parameters of the nonunitary leptonic mixing matrix, thereby testing the origin of $CP$ violation in the lepton sector.