  We study the effect of energy loss on charm and bottom quarks in high-energy
heavy-ion collisions including longitudinal expansion and partial
thermalization. We find that high $p_\perp$ heavy quarks are greatly
suppressed, and consequently, high-mass dileptons from heavy quark decays are
also suppressed. We consider in detail the detector geometry and single lepton
energy cuts of the PHENIX detector at the Relativistic Heavy Ion Collider
(RHIC). Because of the longitudinal expansion, the suppressions of $ee, e\mu$
and $\mu\mu$ pairs resulting from the heavy quark energy loss are very
different due to the different rapidity coverages and energy cuts. The assumed
energy loss rate, on the order of 1 GeV/fm, results in a large suppression on
dielectrons, and dielectrons from heavy quark decays become comparable or even
lower than the Drell-Yan yield. It is thus possible to probe the energy loss
rate of the medium using dileptons from heavy quark decays.
