  General quantum mechanical states of neutrinos produced by mechanisms outside
the Standard Model are discussed. The neutrino state is described by the
Maki-Nakagawa-Sakata-Pontecorvo unitary mixing matrix only in the case of
relativistic neutrinos and Standard Model left-handed charge current
interaction. The problem of Wigner spin rotation caused by Lorentz
transformation from the rest production frame to the laboratory frame is
considered. Moreover, the mixture of the neutrino states as a function of their
energy and parameters from the extension of the Standard Model are
investigated. Two sources of mixture, the appearance of subdominant helicity
states, and mass mixing with several different mixing matrices are studied.
