  We propose a modification of the electroweak theory, where the fermions with
the same electroweak quantum numbers are combined in multiplets and are treated
as different quantum states of a single particle. The developed approach
enables one to calculate the probabilities of the processes taking place in the
detector at long distances from the particle source. Calculations of
higher-order processes including the computation of the contributions due to
radiative corrections can be performed in the framework of the perturbation
theory using the regular diagram technique. As a result, we obtain the analog
to the Dirac--Schwinger equation of quantum electrodynamics describing neutrino
oscillations and its spin rotation in dense matter.
