This question relates to Griffiths: Introduction to Elementary Particles, p. 196 The process in question is a neutral pion decay into two photons. It is stated that because the secondary particles are massless, the amplitude for this process is: [tex] M(p_2,p_3) [/tex] where [tex]p_2[/tex] and [tex]p_3[/tex] are the momentum three-vectors of the secondary photons. Conservation of momentum further implies that [tex]p_2=-p_3[/tex]. So far, so good. However, Griffiths then states that [tex] |M|^2 [/tex] is a function of |p_2| only, meaning that |M|^2 has no angular dependency. Why is this?