On the Nuclear Modification Factor at RHIC and LHC

TL;DR

This paper demonstrates that pQCD factorization with pre-hadronization energy loss explains the flatness of the nuclear modification factor R_{AA} at high transverse momentum in heavy-ion collisions at RHIC and LHC, with two models yielding similar results.

Contribution

It introduces and compares two pre-hadronization energy-loss scenarios, showing their effectiveness in explaining R_{AA} suppression and flatness at high p_T in nucleus-nucleus collisions.

Findings

Flatness of R_{AA} occurs at p_T ≈ 15 GeV at RHIC and 25 GeV at LHC.

Energy-loss models predict similar suppression levels, around 30-50%.

The k_T factorization with BK equation produces R_{AA} < 0.5, largely independent of BK solution details.

Abstract

We show that pQCD factorization incorporated with pre-haronization energy-loss effect naturally leads to flatness of the nuclear modification factor R_{AA} for produced hadrons at high transverse momentum p_T. 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-p_T hadrons in nucleus-nucleus collisions at RHIC. We show that the onset of the flatness of R_{AA} for the produced hadron in central collisions at midrapidity is about p_T\approx 15 and 25 GeV at RHIC and the LHC energies, respectively. We show that the smallness (R_{AA}<0.5) and the high-p_T flatness of R_{AA} obtained from the k_T 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 k_T factorization about 30\div 50% at moderate p_T.