The question is "Make a simple estimate of the order of magnitude of the ratio of the cross sections (electron,positron->2photons) and (electron,positron->3photons)"(adsbygoogle = window.adsbygoogle || []).push({});

Now I know that cross section is proportional to (1/q^4)*(Phase space factor)*(Coupling constant) where q^2 = E^2 - p^2. We are also told that the difference in phase space factors are negligible so will be cancelled in the ratio.

I think I can find coupling constant as g^(number of vertices).

My question is; can you actually find the q^2 with the information I've been given? If not am I meant to assume that it's the same for both, but then the ratio would just equal g which makes no sense. Any help would be really appreciated :)

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# Estimating the order of magnitude of a ratio of two cross sections

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