Where is the charm quark energy loss at RHIC?

Heavy quark energy loss in a hot QCD plasma is computed taking into account the competing effects due to suppression of zeroth order gluon radiation below the plasma frequency and the enhancement of gluon radiation due to first order medium induced Bremsstrahlung. The results suggest a surprising degree of cancellation between the two medium effects for charm quarks and provides a possible explanation for the null effect observed by PHENIX in the prompt electron spectrum in Au+Au at 130 AGeV. Introduction Recent observations [1]-[8] of large suppression moderate p⊥ ∼ 5 GeV hadrons produced in Au + Au at the Relativistic Heavy Ion Collider (RHIC) have been interpreted as evidence for jet quenching of light quark and gluon jets [9]-[16]. Jet quenching was predicted [17, 18] to occur due to medium induced radiative energy loss of high energy partons propagating through ultra-dense QCD matter. The quenching pattern therefore provides a novel tomographic tool that can be used to map out the evolution of the plasma density. Of course, even without final state interactions, gluon radiation associated with hard QCD processes softens considerably the lowest order jet spectra. This is taken into account through the Q (DGLAP) evolution of the hadronic fragmentation functions. Medium induced radiation is the extra gluon radiation that arises from higher twist final state interactions and depends on the optical thickness or opacity L/λ of the medium. A similar quenching pattern was predicted [19, 20, 21] to occur for heavy quark (c or b) jets. However, in [22] it was pointed out that the heavy quark mass leads to a “dead cone” effect for θ < M/E that reduces the induced radiative energy loss of heavy quarks as compared to light partons. Numerical estimates indicated that the quenching of charm quarks may be approximately about a half that of light quarks. Experimentally, PHENIX data [23] on “prompt” single electron production in Au+Au collisions at √ s = 130 AGeV have provided a first test of heavy quark energy loss. Remarkably, no indication for a QCD medium effect was found within the admittedly large experimental errors (see also ref. [24]). In future runs, much higher statistics and wider pT range will become accessible. In this letter we investigate whether the apparent null effect observed for heavy quark energy loss via single electrons could be due to other medium effects that compete with the induced