This question relates to Griffiths: Introduction to Elementary Particles, p. 196(adsbygoogle = window.adsbygoogle || []).push({});

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?

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# Fermi's Golden Rule (Decay Amplitude)

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