Hard Probe of Geometry and Fluctuations in Heavy Ion Collisions at   $\sqrt{s_{_{NN}}}=$ 0.2, 2.76, and 5.5 TeV
  [Background:] A significant quenching of high energy jets was observed in the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) facility, and is now confirmed at the Large Hadron Collider (LHC) facility. The RHIC plus LHC era provides a unique opportunity to study the jet-medium interaction that leads to the jet quenching, and the medium itself at different collision energies (medium temperatures).   [Purpose:] We study the azimuthal anisotropy of jet quenching, to seek constraints on different models featuring distinct path-length and density dependences for jet energy loss, and to gain a better understanding of the medium.   [Methods:] The models are fixed by using the RHIC data, and then applied to study the LHC case. A set of harmonic (Fourier) coefficients $v_{n}$ are extracted from the jet azimuthal anisotropy on a event-by-event basis.   [Results:] The second harmonics $v_{2}$, mostly driven by the medium's geometry, can be used to differentiate jet quenching models. Other harmonics are also compared with the LHC (2.76 TeV) data. The predictions for future LHC (5.5 TeV) run are presented.   [Conclusions:] We find that a too strong path-length dependence (e.g., cubic) is ruled out by the LHC $v_{2}$ data, while the model with a strong near-$T_c$ enhancement for the jet-medium interaction describes the data very well. It is worth pointing out that the latter model expects a less color-opaque medium at LHC. 
Xilin Zhang and Jinfeng Liao
nucl-th hep-ph nucl-ex