Conservation of Helicity and SU(2) Symmetry in First Order Scattering
  The structure of the spin interaction operator (SI) (the interaction that remains after space variables are integrated out) in the first order S-matrix element of the elastic scattering of a Dirac particle in a general helicity-conserving vector potential is investigated.It is shown that the conservation of helicity dictates a specific form of the SI regardless of the explicit form of the vector potential. This SI closes the SU(2) algebra with other two operators in the spin space of the particle. The directions of the momentum transfer vector and the vector bisecting the scattering angle seem to define some sort of "intrinsic" axes at this order that act as some symmetry axes for the whole spin dynamics . The conservation of helicity at this order can be formulated as the invariance of the component of the helicity of the particle along the bisector of the scattering angle in the transition. 
A.Albaid and M.S.Shikakhwa