 Different-flavour quark annihilation into a massive pair

For distinct incoming and outgoing quark flavours, tree-level $q\bar{q}\to Q\bar{Q}$ has one gluon annihilation graph. Averaging the initial colours gives $2/9$, and averaging initial spins gives $1/4$. The outgoing fermion spin sum must use distinct particle and antiparticle projectors. For final mass $M$, $s+t+u=2M^2$ and the displayed result includes all outgoing colour and spin sums. Its amplitude squared remains finite at threshold even though the cross section vanishes through the final-state velocity.