Spinor particle. An indeterminacy in the motion of relativistic   dynamical systems with separately fixed mass and spin
  We give an argument that a broad class of geometric models of spinning relativistic particles with Casimir mass and spin being separately fixed parameters, have indeterminate worldline (while other spinning particles have definite worldline). This paradox suggests that for a consistent description of spinning particles something more general than a worldline concept should be used. As a particular case, we study at the Lagrangian level the Cauchy problem for a spinor particle and then, at the constrained Hamiltonian level, we generalize our result to other particles. 
{\L}ukasz Bratek
math-ph hep-th math.MP quant-ph