I have a homework problem asking whether the decay $\Sigma_0 \to n + \gamma$ is allowed, and if so, we are asked to draw a Feynman diagram for the decay (in the quark level). Clearly the strangeness is violated so the decay must involve weak interaction. It is also very fishy that the strange quark turns into a down quark but I cannot really show, why this decay should not be allowed. However I also cannot draw a Feynman diagram. The single photon seems to be the problem.
When in doubt, look at the particle data group listings .It says it goes 100% to lamda gamma, and gives limits for lamda gamma gamma and lamda e+e-. The reason is that it can decay electromagnetically, because Lamda and Sigma zero have the same quark content, (instead of weakly as Sigma + and Sigma -), and the difference in the couplings is a factor of 10000.
Note also that the s quark decays into an up and a W-. It cannot turn into a down.
The Sigma + decay, to accomodate charges need two hadrons. As seen in the comments a gamma can always be produced, but can one get away from needing two hadrons for charge conservation?
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