On the nuclear modification factor at RHIC and LHC

We show that pQCD factorization incorporated with pre-hadronization energy-loss effect naturally leads to flatness of the nuclear modification factor RAA for produced hadrons at high transverse momentum pT. We consider two possible scenarios for the pre-hadronization: In scenario 1, the produced gluon propagates through dense QCD medium and loses energy. In scenario 2, all gluons first decay to quark-antiquark pairs and then each pair loses energy as propagating through the medium. We show that the estimates of the energy-loss in these two different models lead to very close values and is able to explain the suppression of high-pT hadrons in nucleus-nucleus collisions at RHIC. We show that the onset of the flatness of RAA for the produced hadron in central collisions at midrapidity is about pT≈15 and 25 GeV at RHIC and the LHC energies, respectively. We show that the smallness (RAA<0.5) and the high-pT flatness of RAA obtained from the kT factorization supplemented with the Balitsky-Kovchegov (BK) equation is rather generic and it does not strongly depend on the details of the BK solutions. We show that energy-loss effect reduces the nuclear modification factor obtained from the kT factorization about 30-50% at moderate pT. © 2011 Elsevier B.V.