What is the brightest source for dilepton emissions at ultrarelativistic energies?

We calculate the dilepton emissions as the decay product of the charm and bottom quarks produced in heavy-ion collisions at RHIC energy. We take into account the next-to-leading-order radiative corrections in perturbative QCD to the heavy quark production from an initial hard parton-parton scattering. We find that the thermal charm decay dominates the dilepton production in the low dilepton mass region $(<\mathrm{}2\mathrm{}$ GeV), while the heavy quark production from the initial scattering takes over the intermediate and high mass regions $(>\mathrm{}2\mathrm{}$ GeV). Our result also indicates the importance of the bottom quark in the high mass region $(>\mathrm{}4\mathrm{}$ GeV) due to its large mass and cascade decay. If the initial scattering produced charm suffers a significant energy loss due to the secondary interaction, the bottom decay constitutes the major background for the thermal dileptons.