Constraints on the Path-Length Dependence of Jet Quenching in Nuclear   Collisions at RHIC and LHC
  Recent data on the high-pT pion nuclear modification factor, $R_{AA}(p_T)$, and its elliptic azimuthal asymmetry, $v_2(p_T)$, 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 $\chi^2<2$ level with different temperature-dependent jet-medium couplings and path-length dependence exponents $0\le z \le 2$. We test the sensitivity of predictions to different skewed energy-loss fluctuations via a convenient scaling factor distributed in a finite range $0< \zeta_q < 2+q$ with unit mean. 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 jet-medium coupling. 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]. 
Barbara Betz and Miklos Gyulassy
hep-ph nucl-ex nucl-th