 \left(0 \mp\, \unitvect{p} \cdot \unitvect{p'}\right) \bigl|\vect{p}\bigr| \bigl|\vect{p'}\bigr| \left(\left(E_{\alpha}(\vect{p}) + m_{\alpha}\right)\,\left(E_{\alpha}(\vect{p'}) + m_{\alpha}\right) \pm \bigl|\vect{p}\bigr| \bigl|\vect{p'}\bigr|\right) \, R^{S}_{\mp} \right] \\     &/ \left[ 0 