Saturation of elliptic flow and the transport opacity of the gluon plasma at RHIC

Differential elliptic flow and particle spectra are calculated taking into account the finite transport opacity of the gluon plasma produced in Au + An at E-cm similar to 130 A GeV at RHIC. Covariant numerical solutions of the ultrarelativistic Boltzmann equation are obtained using the MPC parton cascade technique. For typical pQCD (similar to3 mb) elastic cross sections, extreme initial gluon densities, dN(g)/d eta similar to 15 000, are required to reproduce the elliptic flow saturation pattern reported by STAR. However, we show that the solutions depend mainly on the transport opacity, chi = integral dz sigma (1)rho (g), and thus the data can also be reproduced with dN(g)/d eta similar to 1000, but with extreme elastic parton cross sections, similar to 45 mb. We demonstrate that the spectra and elliptic flow, are dominated by numerical artifacts unless parton subdivisions similar to 100-1000 are applied to retain Lorentz covariance for RHIC initial conditions. (C) 2002 Elsevier Science B.V. All rights reserved.