Can dileptons be observed in heavy ion collisions at RHIC?

Both dilepton and charm production at RHIC are considered to be important signatures for Quark-Gluon Plasma production. Recently it was argued by S.Gavin, P.L.McGaughey,P.V. Ruuskanen and R.Vogt [1] that the background from semileptonic correlated charm decays is so large that it makes dilepton measurements virtually impossible. We show that this conclusion in fact is in fact reversed if the energy loss due to secondary interaction of charmed quarks is included. Dileptons produced in highly excited hadronic matter provide valuable information about the hottest and the most dense stages of nuclear collisions. Together with photons, they are the so called penetrating probes [2] which suffer very little secondary interaction. Consequently, dilepton measurements has attracted great deal of attention, both of theorists and experimentalists. Referring specifically to highest energies, let us remind that one of the major RHIC detectors, PHENIX, and ALICE at LHC plan dilepton measurements, both with electron and muon pairs. Another potential QGP signature is thermal production of new quark flavors, especially of charm [2, 4, 5] 1 However, the ordinary partonic production of charm at the first impact is large, and whether it dominates the secondary (and thermal) charm production remains unclear. Nevertheless, charm signal will also be experimentally addressed at RHIC, by STAR and PHENIX collaborations. Since charmed hadrons have substantial semileptonic decays, their simultaneous decay create a ll background for dilepton measurements. This issue was addressed recently by S.Gavin, P.L.McGaughey,P.V. Ruuskanen and R.Vogt (below GMRV) in a detailed paper [1]. Their conclusions are summarized in Fig.1, and they basically imply that the background from leptonic decays of charmed (and even b) quarks is so large, that implementation of the dilepton measurements is virtually impossible in the whole kinematic domain. In this short paper we question those pessimistic conclusions, and suggest that it should be reversed. We show that a very important effect is missing from the GMRV 1 The expected highest temperatures at RHIC [2, 3, 6] are Ti = 400− 500MeV , so the mean energy per parton ≈ 3T is comparable to charm quark mass. Note also that the mass of the strange quark is not large enough to suppress its production in the hadronic phase.