Fermi's Golden Rule (Decay Amplitude)

[getPhysical()]

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:

$$M(p_2,p_3)$$

where $$p_2$$ and $$p_3$$ are the momentum three-vectors of the secondary photons. Conservation of momentum further implies that $$p_2=-p_3$$. So far, so good.

However, Griffiths then states that
$$|M|^2$$
is a function of |p_2| only, meaning that |M|^2 has no angular dependency. Why is this?

 

[Meir Achuz]

The pi zero has spin zero, so its decay cannot have any angular dependence.