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## why should force carriers be only boson?

@bcrowell: regarding "bosons exchanging fermions" and your question
 Quote by bcrowell Could you give an example of the former? I don't see how the spin coupling can work out at the vertices, since you can't couple an integer spin to a half-integer spin to get an integer spin.
In QED you have light-light scattering at one loop level where two photons exchange an electron-positron pair in a 4-fermion box. The reason why we do not call this a "force" is simply b/c it's too weak to be observed directly.
 Hi. Yes, the vacuum loop... Interesting observation. Hm, photon here is scattered off electron-positron pair that briefly emerged from vacuum sea. In such description, photon might be considered a mediator again. I guess so... Looking forward to see where this takes us. Cheers.
 Recognitions: Science Advisor I wouldn't call that a vacuum loop

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 Quote by kurros One can perfectly well have charged scalar particles, however. For instance in the MSSM we have charged scalar Higgses. I think there is still no problem though, since virtual photons are allowed to have unphysical polarisations, i.e. they are allowed to be in the spin-0 longitudinal polarisation state.
I think the solution in the case of charged scalars is simply that the vertices have more than 3 legs. For a vertex with 3 legs, polarization is irrelevant if the spins can't satisfy the triangle inequality.

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 Quote by tom.stoer @bcrowell: regarding "bosons exchanging fermions" and your question In QED you have light-light scattering at one loop level where two photons exchange an electron-positron pair in a 4-fermion box. The reason why we do not call this a "force" is simply b/c it's too weak to be observed directly.
I see what you mean. But obviously we can't have an H-shaped diagram. We need a box, which makes it a higher-order process.

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Hmm right so some of the diagrams have 4 legs, but do they all? If you look at http://arxiv.org/pdf/hep-ph/9902340v3.pdf equation 3.23c you will find the $H^+H^+A^\mu A_\mu$ diagram I guess you are talking about, plus some other fun stuff of a similar nature, but what about the terms in 3.19? Stuff like $H^+ \overleftrightarrow {\partial^\mu} H^- A_\mu$? I don't understand what a term like this means, it seems half kinetic, half interaction term...? Anyway I guess it solves the issue I was having about extra Lorentz indices floating around...