Mass-suppressed effects in heavy quark diffusion

Many lattice studies of heavy quark diffusion originate from a colour-electric
correlator, obtained as a leading term after an expansion in the inverse of the heavy-quark mass. In view of the fact that the charm quark is not particularly heavy, we consider subleading terms in the expansion. Working out correlators up to O(1/M2), we argue that the leading corrections are suppressed by O(T/M), and one of them can be extracted from a colour-magnetic correlator. The corresponding transport coefficient is non-perturbative already at leading order in the weak-coupling expansion, and therefore requires a nonperturbative
determination.