# Difference between 2to2 interactions

by ChrisVer
Tags: 2to2, difference, interactions
 P: 754 What is the difference between these interactions? $a+b \rightarrow c+d$ and $b+a \rightarrow d+c$ ? 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 $M$ forms for these two above.