Dilepton Production from Heavy Quark Decays (with D. Fein, Z. Huang and P. Valerio); Physical Review C56 (1997) 1637.

The emission of dileptons in high energy heavy-ion collisions provides an excellent probe of the property of dense hadronic matter such as the quark-gluon plasma and the hot hadronic gas. Due to the small cross sections of electromagnetic interactions, these dileptons once produced can escape the strong interacting volume of sizes which might be produced in heavy-ion collisions. The dilepton invariant mass spectrum holds the most promising signatures of the quark-gluon plasma such as thermal dileptons and the J/ Psi suppression. However, the use of the dilepton probe is difficult for an obvious reason: it is sensitive to many different sources. In the low mass region, the resonance decays from the light hadrons constitute the main background. In the intermediate and high mass regions, the Drell-Yan dilepton production from the initial hard scattering is important. In high energy heavy-ion colliders such as RHIC and LHC, the heavy flavor quark production can be quite substantial and its subsequent decay can lead to a large combinatorial background for the dilepton spectrum. Therefore it is crucial to make a solid theoretical prediction on the contribution from heavy quark decays.

Recently we have calculated the dilepton emissions as the decay product of the charm and bottom quarks produced in an initial hard scattering and in a thermalized quark-gluon plasma at RHIC energy. We include the next-to-leading order radiative corrections and nuclear shadowing effects. The heavy parton production is calculated using the explicit next-to-leading order matrix elements, while the parton fragmentation and the heavy hadron decay are handled by the Isajet Monte Carlo model. The cascade decay of the bottom quark is also included.

We find that the dimuon spectrum from thermal charm quark production is considerably softer than the initial hard scattering results. However, it dominates the dilepton production in the low mass region (M<2 GeV). This can be a important background at RHIC for the dilepton signature for the low lying vector resonances such as tex2html_wrap_inline497 and tex2html_wrap_inline499 . It is important to note that the calculated thermal charm contribution corresponds to a very ideal case where the quarks and gluons are fully equilibrated. The result may be considerably smaller in the plasma where the chemical process is not in equilibrium. Although the production rate for the initial bottom quark is much smaller than the charm, the dilepton production from B-meson decay is non-negligible in the large invariant mass region due to the large rest mass of the B-meson. It becomes the dominant contribution when the dimuon invariant mass is above tex2html_wrap_inline505 GeV. If the fast charm suffers the significant energy loss before it hadronizes, the dimuon spectrum from the initial fusion charm can be considerably softened. In fact, it becomes very similar to that of the thermal charm and falls off rapidly as M gets bigger than 2 GeV. In this case, the bottom dominance is more prominent and starts at a smaller value of tex2html_wrap_inline509 GeV because two B-mesons at rest can produce an energetic lepton pair.