electromagnetic radiation - 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.

Answer

A good paper to look at is this. It lists and describes all the reactions that you can imagine involving A=4 and thus the 4He system. Fig. 2 on p.16 is interesting. I'm still trying to understand what the plots of the excitation function are. It's also worth noting that this paper says that no excited state of 4He is bound.

Perhaps the most interesting part is Table 3.0.1 on p.19. Here gamma decay modes are shown, though some only in bracket. I didn't expect to see a gamma channel in the 20.21MeV and 21.01MeV states since Delta J = 0. For the 21.84MeV and 23.33MeV states also I didn't expect a large channel since Delta J = 2 and E2 is always a difficult mode. What stuck out to me was why the 23.64MeV has (gamma) in brackets (I don't know that means just yet) but 24.25MeV does not! So I did a search and found this:

In the single particle estimates of gamma decay, one presumes a single nucleon interacts with a photon. This means there is an isospin selection rule Delta T = 0 or 1 for gamma decay between two pure isospin states. Also E1 cannot occur when Delta T = 0 in a self-conjugate nucleus (N = Z).

This means that the 24.25MeV level can not make a E1 transition, hence why it is suppressed.

The next possible E1 transitions are the 25.95MeV and 28.67MeV levels. Listed next to it are the reactions where there are more info.