Scalar and vector interactions, with the scalar interaction coupled to a composite spin-1/2 system so as to cause a shift of its mass, are shown to obey a low-energy theorem which guarantees that the second order interaction is the same as for a point Dirac particle, in which case the second-order interaction comes from z graphs. Off-shell and contact interactions appropriate to the composite system cancel and this is verified in a model of a composite fermion. The low-energy theorem and its generalizations provide a justification for the use of the Dirac equation as it has been used in relativistic nuclear scattering and mean field theories. © 1996 The American Physical Society.

• Received 22 August 1995

DOI:https://doi.org/10.1103/PhysRevC.53.860