= Low-energy charged-scalar scattering through a neutral scalar
{title2=$\mathcal M\simeq2g^2/m^2$}
For interaction $-g\varphi^\dagger\varphi\phi$ with a real mediator of mass $m$, fixed-charge particle–antiparticle scattering has $s$- and $t$-channel exchange. At $|s|,|t|\ll m^2$, their amplitudes add to $2g^2/m^2$. Distinguishable massless final particles then give $\sigma\simeq g^4/(4\pi m^4s)$. There is no extra $u$ channel for the same fixed assignment of final charges.
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