Two-Body Dirac Equation and Its Wave Function at the Origin

A relativistic equation is deduced for the bound state of two particles, by assuming a proper boundary condition for the propagation of the negative-energy states. It reduces to the (one-body)Dirac equation in the infinite limit of one of the constituent mass. It also has the symmetries to assure the existence of the anti-bound-state with the same mass. The interaction kernel(pseudo-potential) is systematically constructed by diagonalizing the Hamiltonian of the background field theory, by which the retardation effects are included in the interaction. Its wave function at the origin(WFO) behaves properly in a manner suggested by the covariant field theory. The equation can well be applied in the heavy quark effective theory. It can also be used as a starting equation for the unperturbed system in the rigorous perturbation theory of QED.