What is the difference between these interactions? [itex] a+b \rightarrow c+d[/itex] and [itex] b+a \rightarrow d+c [/itex] ? I was reading the "State-of-the-art formulas for helicity amplitude calculation and all that" ver 2.4, part Ia. - Spherical-vector method for helicity amplitudes (Formalism), by Ken-Ichi Hikasa. In the discrete symmetry and helicity amplitudes sector (pg 5) he gives different invariant matrices [itex]M[/itex] forms for these two above.