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Heavy quark chemical equilibration rate as a transport coefficient

Motivated by indications that heavy (charm and bottom) quarks interact strongly at temperatures generated in heavy ion collision experiments, we suggest a non-perturbative definition of a heavy quark chemical equilibration rate as a transport coefficient. Within leading-order perturbation theory (corresponding to 3-loop level), the definition is argued to reduce to an expression obtained from the Boltzmann equation. Around T ~ 400 MeV, an order-of-magnitude estimate for charm yields a rate Gamma^{-1}_{chem} > 60 fm/c which remains too slow to play a practical role in current experiments. However, the rate increases rapidly with T and, due to non-linear effects, also if the initial state contains an overabundance of heavy quarks.