The Surprising Transparency of the sQGP at LHC

TL;DR

This paper predicts jet quenching effects in Pb+Pb collisions at LHC energies using a parameter-free model, compares predictions with early ALICE data, and discusses discrepancies that challenge basic assumptions about QGP opacity.

Contribution

It provides the first parameter-free predictions of high p_T pion suppression at LHC based on RHIC data constraints and examines the validity of jet quenching assumptions at higher energies.

Findings

Predicted significant differences in jet quenching between RHIC and LHC energies.

Over-quenching observed in pion R_{cp} compared to ALICE data.

Discrepancy questions the linear scaling of energy loss with QGP density.

Abstract

We present parameter-free predictions of the nuclear modification factor, R_{AA}^pi(p_T,s), of high p_T pions produced in Pb+Pb collisions at sqrt{s}_{NN}=2.76 and 5.5 ATeV based on the WHDG/DGLV (radiative+elastic+geometric fluctuation) jet energy loss model. The initial quark gluon plasma (QGP) density at LHC is constrained from a rigorous statistical analysis of PHENIX/RHIC pi^0 quenching data at sqrt{s}_{NN}=0.2 ATeV and the charged particle multiplicity at ALICE/LHC at 2.76 ATeV. Our perturbative QCD tomographic theory predicts significant differences between jet quenching at RHIC and LHC energies, which are qualitatively consistent with the p_T-dependence and normalization---within the large systematic uncertainty---of the first charged hadron nuclear modification factor, R^{ch}_{AA}, data measured by ALICE. However, our constrained prediction of the central to peripheral pion modification, R^pi_{cp}(p_T), for which large systematic uncertainties associated with unmeasured p+p reference data cancel, is found to be over-quenched relative to the charged hadron ALICE R^{ch}_{cp} data in the range 5<p_T<20 GeV/c. The discrepancy challenges the two most basic jet tomographic assumptions: (1) that the energy loss scales linearly with the initial local comoving QGP density, rho_0, and (2) that \rho_0 \propto dN^{ch}(s,C)/dy is proportional to the observed global charged particle multiplicity per unit rapidity as a function of sqrt{s} and centrality class, C. Future LHC identified (h=pi,K,p) hadron R^h_{AA} data (together with precise p+p, p+Pb, and Z boson and direct photon Pb+Pb control data) are needed to assess if the QGP produced at LHC is indeed less opaque to jets than predicted by constrained extrapolations from RHIC.