= Tree scattering with pseudoscalar exchange
{title2=$V=-ig\gamma^5$}
The <pseudoscalar Yukawa interaction> with coefficient $-g$ has the displayed <Feynman vertex>. Its two-fermion <tree-level Feynman diagrams> are the $t$ and $u$ exchanges; antisymmetry of the final identical fermions requires their relative minus. Fermion-antifermion scattering has $t$ exchange and $s$ annihilation, again with a relative minus. With initial and final states ordered as $b_1^\dagger d_2^\dagger|0\rangle$ and $b_3^\dagger d_4^\dagger|0\rangle$, the normal-ordered current contributes $-d^\dagger d\,\bar v\gamma^5v$ in the exchange channel but $db$ and $b^\dagger d^\dagger$ in the annihilation channel. These <fermionic signs> fix the overall sign as well as their interference; changing an external-state phase changes both terms together.
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