Constraints on the path-length dependence of jet quenching in nuclear collisions at RHIC and LHC

Recent data on the high-p T pion nuclear modification factor, R-AA (pT), and its elliptic azimuthal asymmetry, v(2) (pT), from RHIC/BNL and LHC/CERN are analyzed in terms of a wide class of jet-energy loss models coupled to different (2+1)d transverse plus Bjorken expanding hydrodynamic fields. We test the consistency of each model by demanding a simultaneous account of the azimuthal, the transverse momentum, and the centrality dependence of the data at both 0.2 and 2.76 ATeV energies. We find a rather broad class of jet-energy independent energy-loss models dE/dx = kappa(T)x(z)T(2+z)zeta q that, when coupled to bulk constrained temperature fields T(x, t), can account for the current data at the chi(2)/d.o.f. <2 level with different temperature-dependent jet-medium couplings, kappa(T), and path-length dependence exponents 0 <= z <= 2. We extend previous studies by including a generic term, 0 < zeta(q) < 2 + q, to test different scenarios of energy-loss fluctuations. While a previously proposed AdS/CFT jet-energy loss model with a temperature-independent jet-medium coupling as well as a near-T-c dominated, pQCD-inspired energy-loss scenario are shown to be inconsistent with the LHC data, once the parameters are constrained by fitting to RHIC results, we find several new solutions with a temperature-dependent kappa(T). We conclude that the current level of statistical and systematic uncertainties of the measured data does not allow a constraint on the path-length exponent z to a range narrower than [0 -2].