= Solution
Let $p_1,p_3$ label the incoming and outgoing <fermion>, and $p_2,p_4$ the incoming and outgoing <antifermion>. The bottom row of the figure gives the two <tree-level Feynman diagrams>:
* The $t$ channel has an open <fermion> line $1\to3$ and an open <antifermion> line with <fermion flow> $4\to2$. The <scalar propagator> carries $q_t=p_1-p_3=p_4-p_2$.
* The $s$ channel has initial <particle-antiparticle annihilation> into a <scalar propagator> with $q_s=p_1+p_2=p_3+p_4$, followed by creation of the final pair.
Thus the <Mandelstam variables> in the two denominators are
$$
\boxed{t=(p_1-p_3)^2,\qquad s=(p_1+p_2)^2.}
$$
The two <scattering amplitude> terms have a relative <fermionic sign> of minus when written with spinor products $(\bar u_3u_1)(\bar v_2v_4)$ and $(\bar v_2u_1)(\bar u_3v_4)$. The overall phase depends on the chosen ordering of the external states. No $u$ channel joins an incoming <fermion> to an outgoing <antifermion> through a species-preserving <Yukawa interaction>.
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