Relativistic two-particle problem in the Lagrange formalism

Abstract: The relativistic two body problem is considered in terms of the action integral in the case of two interacting spinless particles and spin-$1/2$ fermions, interacting by means of vector and scalar fields. The Lagrangians governing the dynamics of such particles are derived in the momentum representation. In the Breit frame, when the two-body system is at rest as whole, the obtained Lagrangians generate the Klein-Gordon-like and Dirac-like equations in the (3+1) phase space. The derived equations are examined by studying a para-positronium in a strong magnetic field. It is shown that a strong magnetic fields result in essential decreasing the para-positronium life-time.