Solution
= Solution
For $\Psi(p_1)\to\chi(p_2)+\phi(p_3)$, the sole <tree-level Feynman diagram> is a single flavour-changing <Yukawa coupling> <interaction vertex>, with $p_1=p_2+p_3$. Its invariant <decay amplitude> is
$$
\boxed{i\mathcal M_{s_1s_2}=-ig\,\bar u_\chi^{s_2}(p_2)u_\Psi^{s_1}(p_1).}
$$
The solid arrow passes from the incoming $\Psi$ to the outgoing $\chi$; the dashed outgoing line carries $p_3$.