K/π Probes of Jet Quenching in AA Collisions

Non-abelian energy loss in quark gluon plasma is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. We apply recent (GLV) estimates for the gluon radiative energy loss, which increases linearly with the jet energy up to E < 20 GeV and depends quadratically on the nuclear radius, R. The K/π ratio is found to be most sensitive to the initial density of the plasma. Energy loss of high energy quark and gluon jets penetrating dense matter produced in ultrarelativistic heavy ion collisions leads to jet quenching and thus probes the quark-gluon plasma formed in those reactions [1, 2]. The non-abelian radiative energy loss, ∆E(E,L), suppresses the moderate 2 − 3 GeV < pT < 10 − 15 GeV distributions of hadrons in a way that can also influence the jet fragmentation pattern into different flavor hadrons. This is because quark and gluon jets suffer different energy losses proportional to their color Casimir factors (4/3, 3). First estimates [1, 2] suggested that ∆E ≈ 1−2 GeV(L/fm) would depend linearly on the plasma thickness, L, as in abelian electrodynamics. In BDMS [3] non-abelian (radiated gluon final state interaction) effects were shown to lead to a quadratic dependence on L with a much larger magnitude of ∆E. The analysis of jet-quenching in Pb+Pb at √ s = 20 AGeV, however, indicated a negligible energy loss at that energy [4, 5]. In GLV [6, 7] finite kinematic constraints were found to reduce greatly the energy loss at moderate jet energies and we obtained ∆E(E,L) ∼ E · (L/6 fm), Here we apply the GLV energy loss to estimate more quantitatively the effect of jet quenching on the hadronic ratios at moderate p⊥ < 10 GeV at the Relativistic Heavy Ion Collider (RHIC) energy, √ s = 130 AGeV. We illustrate the jet quenching effects for a generic plasma with an average screening scale μ = 0.5 GeV, strong coupling αs = 0.3, and an average gluon mean free path λg = 1 fm. We introduce the opacity as the mean number of jet scatterings, n̄ = L/λ, which is assumed to be a finite number. An advantage of looking into particle ratios is that uncertainties in the absolute normalization due to acceptance tend to cancel. The disadvantage is of course that high p⊥ particle identification is increasingly difficult. As we have shown in Ref. [8], the kinematically suppressed GLV energy loss turns out to depend approximately linearly on the jet energy, E. This linear dependence, ∆E ∝ E, leads to only a very weakly p⊥ dependent suppression of the transverse momentum distributions for the kinematic range accessible experimentally at RHIC. We focus on the p⊥ dependence K/π Probes of Jet Quenching in AA Collisions 2 of the particle ratios in order to help pin down the jet quenching mechanism. Our primary candidate is the measurable K/π ratio. In order to investigate the influence of the GLV energy-dependent radiative energy loss on hadron production, we apply a perturbative QCD (pQCD) based description of Au + Au collisions, including energy loss prior to hadronization. First, we check that the applied pQCD description reproduces data on pion and kaon production in p+p collision. Our results are based on a leading order (LO) pQCD analysis. Detailed discussion of the formalism is published elsewhere [9, 10]. Our pQCD calculations incorporate the parton transverse momentum (“intrinsic kT ”) via a Gaussian transverse momentum distribution g(~kT ) (characterized by the width 〈k2 T 〉) [9, 10, 11] with the usual convolution of the parton distribution functions (PDF) fa/p, partonic cross sections and fragmentation functions (FF) Dh/c, as: