Saturation of Elliptic Flow and the Transport Opacity of the Gluon Plasma at RHIC

Denes Molnar, Miklos Gyulassy

Published 2001-04-24, updated 2003-12-03Version 3

Differential elliptic flow and particle spectra are calculated taking into account the finite transport opacity of the gluon plasma produced in Au+Au at Ecm ~ 130 A GeV at RHIC. Covariant numerical solutions of the ultrarelativistic Boltzmann equation are obtained using the MPC parton cascade technique. For typical pQCD (~3 mb) elastic cross sections, extreme initial gluon densities, dN/deta ~ 15000, are required to reproduce the elliptic flow saturation pattern reported by STAR. However, we show that the solutions depend mainly on the transport opacity, $\chi=\int dz \sigma_t\rho_g$, and thus the data can also be reproduced with dN/deta ~ 1000, but with extreme elastic parton cross sections, \~45 mb. We demonstrate that the spectra and elliptic flow are dominated by numerical artifacts unless parton subdivisions ~100-1000 are applied to retain Lorentz covariance for RHIC initial conditions.