Feynman diagram for radiative transition

[kuecken]

Hi,
I was just trying to draw a Feynman diagram for ψ' to χ_{c} states.
So we have a electromagnetic transition.

I feel tempted to just have c and \bar{c} coming in and letting a γ leave from one of the legs. But that looks like an incomplete Feynman diagram to me. Maybe I have to get two photons out?

Can someone help me with this please!

 

[mfb]

You need as many photons as there are photons in the final state. Feynman diagrams for electromagnetic decay of hadrons are a bit boring, as the interesting part happens in the QCD binding which you cannot include in that way.