  We discuss the average collisional energy loss dE/dx of a heavy quark
crossing a quark-gluon plasma, in the limit of high quark energy E >> M^2/T,
where M is the quark mass and T >> M is the plasma temperature. In the fixed
coupling approximation, at leading order dE/dx \propto \alpha_s^2, with a
coefficient which is logarithmically enhanced. The soft logarithm arising from
t-channel scattering off thermal partons is well-known, but a collinear
logarithm from u-channel exchange had previously been overlooked. We also
determine the constant beyond those leading logarithms. We then generalize our
calculation of dE/dx to the case of running coupling. We estimate the remaining
theoretical uncertainty of dE/dx, which turns out to be quite large under RHIC
conditions. Finally, we point out an approximate relation between dE/dx and the
QCD Debye mass, from which we derive an upper bound to dE/dx for all quark
energies.
