  There are confusions about angular momentum propagation in scattering or
decay processes involving the transition between particle systems that appear
to transform differently under Lorentz transformations. This paper provides an
analysis of the transformation properties of the states and interactions for a
few typical processes within the standard model of particle physics, and
performs explicit calculations showing how angular momentum transfers in these
processes. We shall show that angular momentum conservation is completely
consistent with the calculation in quantum field theory.
