Viscous Flow in Heavy-Ion Collisions from RHIC to LHC

We present a systematic hydrodynamic study of the evolution of hadron spectra and their azimuthal anisotropy from the lowest collision energy studied at the Relativistic Heavy Ion Collider (RHIC), s=7.7A GeV, to the highest energy reachable at the Large Hadron Collider (LHC), s=5500A GeV [C. Shen and U. Heinz, Phys. Rev. C 85, 054902 (2012) [arXiv:1202.6620 [nucl-th]].]. The energy dependence of the flow observables are quantitatively studied for both the Monte-Carlo Glauber and Monte-Carlo Kharzeev-Levin-Nardi (MC-KLN) models. For MC-Glauber model initial conditions with η/s=0.08, the differential charged hadron elliptic flow v2ch(pT,s) is found to exhibit a very broad maximum in the region 39⩽s⩽2760A GeV. For MC-KLN initial conditions with η/s=0.2, a similar “saturation” is not observed up to LHC energies. We emphasize that this “saturation” of elliptic flow arises from the interplay between radial flow and elliptic flow which shifts with s depending on the fluidʼs viscosity. By generalizing the definition of spatial eccentricity to isothermal hyper-surface, we also calculate εx on the kinetic freeze-out surface at different collision energies.