  We analyze the causal action principle for a system of relativistic fermions
composed of massive Dirac particles and neutrinos. In the continuum limit, we
obtain an effective interaction described by a left-handed, massive SU(2) gauge
field and a gravitational field. The off-diagonal gauge potentials involve a
unitary mixing matrix, which is similar to the Maki-Nakagawa-Sakata matrix in
the standard model.
