= Charge-two cubic complex-scalar interaction
{title2=$\mathcal L_I=-g(\phi^2\Phi^\dagger+\phi^{\dagger2}\Phi)/2$}
This interaction preserves the <global U(1) symmetry> with relative charges $q_\phi=1$, $q_\Phi=2$. The two conjugate <interaction vertices> have <Feynman rule> $-ig$, with either one charge-two arrow entering and two charge-one arrows leaving, or all arrows reversed. For $\phi\phi\to\phi\phi$ only the heavy $s$-channel exchange occurs at tree level. For $\phi\bar\phi\to\phi\bar\phi$, with outgoing particle labelled 3 and outgoing antiparticle labelled 4, only the heavy $u$-channel exchange occurs. For small momenta and heavy mediator mass, the amplitudes coincide but the identical-particle total <scattering cross-section> is smaller by the <identical final-state symmetry factor> $1/2$.
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