The Salpeter equation and probability current in the relativistic   Hamiltonian quantum mechanics
  The probablity current for a quantum spinless relativistic particle is introduced based on the Hamiltonian dynamics approach utilizing the Salpeter equation as an alternative of the Klein-Gordon equation. The correctness of the presented formalism is illustrated by examples of exact solutions to the Salpeter equation including the new ones introduced in this work. 
K. Kowalski and J. Rembielinski
math-ph hep-ph math.MP