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## Main Question or Discussion Point

According to Tong there are two Feynman diagrams for nucleon-nucleon scattering in the interaction of the complex and real scalar fields, but I can draw another diagram where the p1 and p2 particles enter a vertex and the p1' and p2' particles go out of another vertex (linked to the first one by a dotted real-field line, of course). Why is this diagram impossible?

I think it is because the interaction term is ψ*ψΦ, so every vertex requires a real field particle plus either a particle going in and an antiparticle going out or an antiparticle-particle pair both going out or going in. Is that so?

I think it is because the interaction term is ψ*ψΦ, so every vertex requires a real field particle plus either a particle going in and an antiparticle going out or an antiparticle-particle pair both going out or going in. Is that so?

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