LOW-ENERGY THEOREM FOR SCALAR AND VECTOR INTERACTIONS OF A COMPOSITE SPIN-1/2 SYSTEM

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 due to Z graphs is the same as for a point Dirac particle. Off-shell and contact interactions appropriate to the composite system cancel, and this is verified in a model of a composite fermion. The result suggests a justification for the use of the Dirac equation as it has been used in relativistic nuclear scattering and mean field theories. © 1995 The American Physical Society.