Parity violation and modification of fermion spin projectors

Abstract

Abstract Account of interactions in frameworks of Standard Model leads to appearence of γ5 terms in a dressed fermion propagator. In this case the standard spin projectors do not commute with propagator and should be modified. Starting point of our discussion is the eigenvalue problem for inverse propagator S(p). Having the solutions of this problem, one can understand the necessary modification of spin projectors. The same generalized spin projector appears at propagation of neutrino in a moving matter, but in contrast to vacuum case, corresponding polarization vector is not arbitrary. This spin projector commute with neutrino propagator and all dispersion laws for neutrino in media are classified according to spin projection onto this fixed axis.