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I'm dealing with a doubly charged scalar singlet that interacts only with the right-handed muon as follows,

$$\mathcal{L} = \lambda \psi_{R}C\psi_{R} \phi^{++},$$

where $\lambda$ is the coupling, $\psi_{R}$ is the right-handed muon, $C$ is the charge conjugation matrix in some representation and $\phi^{++}$ is the doubly charged scalar singlet. My question is, how to extract Feyman's rules from this? More precisely, the vertex.

My first thought was: $i\lambda \frac{1}{2}(1+\gamma_{5})$C, but when I think about fermions interacting with a scalar I'm supposing something like $\mathcal{L} = \lambda \overline{\psi}\psi\phi$, from where the vertex is $i\lambda$. But I don't have an adjoint $\overline{\psi}$, and I have no idea how the charge conjugation will get in the vertex expression.

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