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 a t midrapidity of the elliptic flow coefficient nu (2) is calculated in the hydrodynamic and low density limits. Hydrodynamics is found to agree well w ith 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 S PS energies. The low density limit LDL is inconsistent with the measured ma gnitude of nu (2) at RHIC energies and with the shape of its p(t)-dependenc e at both RHIC and SPS energies. The success of the hydrodynamic model poin ts 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.