PROPAGATORS AND PATH-INTEGRALS

Path-integral expressions for one-particle propagators in scalar and f ermionic field theories are derived, for arbitrary mass. This establis hes a direct connection between field theory and specific classical po int-particle models. The role of world-line reparametrization invarian ce of the classical action and the implementation of the corresponding BRST symmetry in the quantum theory are discussed. The presence of cl assical world-line supersymmetry is shown to lead to an unwanted doubl ing of states for massive spin-1/2 particles. The origin of this pheno menon is traced to a ''hidden'' topological fermionic excitation. A di fferent formulation of the pseudo-classical mechanics using a bosonic representation of ys is shown to remove these extra states at the expe nse of losing manifest supersymmetry.