Propagator for a spin 1/2 particle in terms of the unitary   representations of the Lorentz group
  In this paper we extend our previous result on the description of the partcle motion in a generalized Heisenberg picture to a relativistic fermion. The operators of the Lorentz algebra in this picture may be regarded as field operators. In this approach the transition amplitudes for the particle are constructed in terms of two-component functions in the unitary representations of the Lorentz group. 
Rudolf A. Frick