  The chemical equilibration of heavy quarks in a quark-gluon plasma proceeds
via annihilation or pair creation. For temperatures T much below the heavy
quark mass M, when kinetically equilibrated heavy quarks move very slowly, the
annihilation in the colour singlet channel is enhanced because the quark and
antiquark attract each other which increases their probability to meet, whereas
the octet contribution is suppressed. This is the so-called Sommerfeld effect.
It has not been taken into account in previous calculations of the chemical
equilibration rate, which are therefore incomplete for T < alphas^2 M. We
compute the leading-order equilibration rate in this regime; there is a large
enhancement in the singlet channel, but the rate is dominated by the octet
channel, and therefore the total effect is small. In the course of the
computation we demonstrate how operators that represent the annihilation of
heavy quarks in non-relativistic QCD can be incorporated into the
imaginary-time formalism.
