# Why do excited states in $^4$He not decay by photon emission?

Here's a level scheme for the $^4$He nucleus (source; click image to see full size):

Notice that all of the confirmed decay modes are by disintegration — emission of a neutron, proton, or deuteron. There are a few states with the right spin-parity to decay to the ground state by an E1 (from $1^-$) or E2 (from $2^+$) electromagnetic transmission, but there seems to be no evidence that those internal transitions have been observed.

Is there a simple reason why electromagnetic transitions are so strongly disfavored?

If I were giving a talk and this question came up I would mumble something about the strong force winning out over electromagnetism in few-nucleon systems and get back on topic, but I'd like a slightly more sophisticated answer than that.

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This arose in discussion of physics.stackexchange.com/q/111393/44126 – rob May 7 '14 at 20:53