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Fermi's Golden Rule (Decay Amplitude)

  1. May 9, 2010 #1
    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:


    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
    is a function of |p_2| only, meaning that |M|^2 has no angular dependency. Why is this?
    Last edited: May 9, 2010
  2. jcsd
  3. May 10, 2010 #2


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