Low-energy interaction of composite spin-half systems with scalar and   vector fields
  We consider a composite spin-half particle moving in spatially-varying scalar and vector fields. The vector field is assumed to couple to a conserved charge, but no assumption is made about either the structure of the composite or its coupling to the scalar field. A general form for the piece of the spin-orbit interaction of the composite with the scalar and vector fields which is first-order in momentum transfer ${\bf Q}$ and second-order in the fields is derived. 
D.R. Phillips (University of Maryland), M.C. Birse (University of   Maryland and University of Manchester), and S.J. Wallace...