= Quartic Yukawa interaction in three spacetime dimensions
{title2=$-\tfrac g2\phi^2\bar\psi\psi$}
A nonderivative interaction of two real scalar legs with a fermion pair in $2+1$ dimensions. The <engineering dimensions of relativistic fields> are $[\phi]=1/2$ and $[\psi]=1$, so $g$ is a <marginal coupling> by classical <power counting>. The internal symmetry $\phi\mapsto-\phi$ forbids odd-scalar interactions, but allows a sextic scalar interaction. The two-component <Dirac field> representation must be distinguished from four-component <gamma matrices> in four spacetime dimensions.
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