Collisional energy loss of a fast heavy quark in a quark-gluon plasma

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 proportional to alpha(2)(s), 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 running coupling. We estimate the remaining theoretical uncertainty of dE/dx, which turns out to be quite large under Relativistic Heavy Ion Collider 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.