= Different-flavour quark annihilation into a massive pair
{title2=$\overline{|T|^2}=\frac{4g_s^4}{9s^2}[(M^2-t)^2+(M^2-u)^2+2M^2s]$}
For distinct incoming and outgoing <Quark flavour>[quark flavours], tree-level $q\bar q\to Q\bar Q$ has one <gluon> annihilation graph. Averaging the initial <Color charge>[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 <Color charge>[colour] and <spin sums>. Its <Scattering amplitude>[amplitude] squared remains finite at <Particle-production threshold>[threshold] even though the <cross section> vanishes through the final-state <velocity>.
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