Elliptic flow at SPS and RHIC: from kinetic transport to hydrodynamics

Anisotropic transverse flow is studied in Pb + Pb and Au + Au collisions at SPS and RHIC energies, The centrality and transverse momentum dependence at midrapidity of the elliptic flow coefficient nu (2) is calculated in the hydrodynamic and low density limits. Hydrodynamics is found to agree well with the RHIC data for semicentral collisions up to transverse momenta of 1-1.5 GeV/c, but it considerably overestimates the measured elliptic how at SPS energies. The low density limit LDL is inconsistent with the measured magnitude of nu (2) at RHIC energies and with the shape of its p(t)-dependence at both RHIC and SPS energies. The success of the hydrodynamic model points to very rapid thermalization in Au + Au collisions at RHIC and provides a serious challenge for kinetic approaches based on classical scattering of on-shell particles. (C) 2001 Published by Elsevier Science B.V.