  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.
