I'm trying to draw Feynman diagrams for 'The decay of the pseudoscalar meson $\phi = |s \bar{s}\rangle $ into these final states:

i) $\phi \rightarrow \pi^{+} \pi^{-},$ where $\pi^{+}=|u \bar{d}\rangle$ and $\pi^{-}=|d \bar{u}\rangle$

ii) $\phi \rightarrow \pi^{+} \pi^{-} \gamma$ '

but when I was reading up how to do them, I came across this from David Griffin's 'Introduction to Elementary Particles' which seems to explicitily say that the reaction isn't possible. This tarries with everything else I've found on the web: the ϕ doesn't decay into just two two pions. Is there an error in the question ? If not, how I can go about attacking how to draw the Feynman diagrams ? Taken from Page 75, David Griffins

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    $\begingroup$ Greetings! Your shortened version of the question invalidated the existing answer, so I have restored it. $\endgroup$
    – rob
    Jun 14, 2020 at 16:53

1 Answer 1


A quick trip to the PDG will reassure you that reaction i) goes at a BR of $7\cdot 10^{-5}$ and reaction ii) at $4\cdot 10^{-5}$.

What Griffiths' book calls "forbidden" means "greatly suppressed", and I assume you went where he is sending you — but I am not familiar with that text.

In the strong interactions, I and G are pretty good symmetries, so the φ having negative G cannot go strongly to two pions each with G=-, so collectively with G=+.

Nevertheless, electromagnetism breaks I and hence G, and an exchanged photon costs an α in the amplitude and the square of that in the rate. Do you now see where the suppression factor in the BR must be coming from? What do your Feynman diagrams look like?

  • $\begingroup$ Thanks- that's reassuring ! I've added what I think the Feynman diagram is, but I'm still not sure where the photon is radiated from. $\endgroup$
    – user264724
    May 21, 2020 at 0:57
  • $\begingroup$ From any quark; for i), from any quark to any other, more or less. $\endgroup$ May 21, 2020 at 2:44
  • $\begingroup$ Thank-You, does the Feynman diagram look correct to you? Thank-You so much! $\endgroup$
    – user264724
    May 21, 2020 at 2:54
  • $\begingroup$ Doesn’t look wrong, only funny: The photon could originate in the middle of a quark line. $\endgroup$ May 21, 2020 at 13:20

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