New explicit expressions for Dirac bilinears

We derive new explicit expressions for the Dirac bilinears based on a generic representation of the massive Dirac spinors with canonical polarization. These bilinears depend on a direction n in Minkowski space which specifies the form of dynamics. We argue that such a dependence is unavoidable in a relativistic theory with spin, since it originates from Wigner rotation effects. Contrary to most of the expressions found in the literature, ours are valid for all momenta and canonical polarizations of the spinors. As a byproduct, we also obtain a generic explicit expression for the covariant spin vector.