The coannihilation rate of kinetically equilibrated non-relativistic
particles plays an essential role in the classic WIMP dark matter
scenario. If the dark matter particles interact attractively, for
instance through Z0 exchange, the coannihilation rate could be
substantially increased with respect to a tree-level estimate, a
phenomenon known as the Sommerfeld effect. We study this physics in an
analogous QCD situation, replacing WIMPs by charm or bottom quarks at
a temperature of a few hundred MeV. Through resummed perturbative
estimates and lattice simulations, we demonstrate that the
coannihilation rate can increase much more than predicted by the
standard Sommerfeld enhancement, by up to two orders of magnitude.
The boost originates from the formation of (thermally broadened)
bound states, which subsequently decay.