Effect of the ``minimal'' viscosity on observables at RHIC

The goal of heavy ion experiments at the Relativistic Heavy Ion Collider (RHIC) is to create and study a novel hot and dense phase of quark-gluon matter, the so-called quark gluon plasma (QGP). Several features of the RHIC data can be reproduced using ideal hydrodynamics, which lead to the suggestion that the plasma could be a ``perfect fluid.'' However, ideal hydrodynamics assumes zero viscosity (and therefore no dissipation), contrary to general expectations based on quantum mechanics that imply finite rates and, therefore, a nonzero ``minimal viscosity.'' These expectations have been verified for strongly-coupled ${\cal N}=4$ supersymmetric Yang-Mills theories, for which the lower bound on the shear viscosity to entropy density ratio is $\eta/s = 1/(4\pi)$. Parton kinetic theory calculations based on microscopic $2\to 2$ rates do indicate that short, but non-zero, effective mean free paths generate sizable dissipative effects for conditions expected at RHIC. We show that these results imply that even a small $\eta/s$ ratio $\sim 0.1$ affects observables at RHIC and leads to significant deviations from ideal hydrodynamic behavior.