Thread: Photon Decay
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Jul26-11, 08:31 PM
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Quote Quote by -dove View Post
-So I guess my first question is, is this reasoning correct and does it prove that QED does not allow for a single photon to decay into two photons?
You don't have to depend on those of us who participated in that thread to have gotten that right. The Fiore paper confirms it.

Quote Quote by -dove View Post
The thread that I linked made it sound like all photon decays are forbidden. However, the cited paper was a little over my head. So can anyone explain why the following diagram is not allowed?
This is the case that we were all frustrated with. Fiore offers technical arguments to the effect that it's impossible, but all of us felt that it should be possible to rule it out on more straightforward grounds. If those fermions in your diagram are massive, then this post by PAllen offers an argument against the decay that I personally find persuasive.

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Re your question about [itex]2\gamma\rightarrow4\gamma[/itex], I don't see any reason why it couldn't happen. There isn't any requirement for the decay products to be collinear as there is in [itex]\gamma\rightarrow3\gamma[/itex], so the phase-space volume doesn't vanish. But it has six vertices, so I would expect it to be very low in cross-section compared to [itex]2\gamma\rightarrow2\gamma[/itex].