Kinematical theory of spinning particles: The interaction Lagrangian for   two spin 1/2 Dirac particles
  The concept of elementary particle rests on the idea that it is a physical system with no excited states, so that all possible states of the particle are just kinematical modifications of any one of them. In this way instead of describing the particle attributes it amounts to describe the collection of consecutive inertial observers who describe the particle in the same kinematical state. The kinematical state space of an elementary particle is a homogeneous space of the kinematical group.By considering the largest homogeneous spaces of both, Galilei and Poincare groups, it is shown how the spin structure is related to the different degrees of freedom. Finally, the spacetime symmetry group of a relativistic particle which satisfies Dirac's equation when quantized, is enlarged to take into account additional symmetries like spacetime dilations and local rotations. An interaction Lagrangian invariant under this enlarged group is proposed and the compound system of two Dirac particles is analyzed. 
Martin Rivas