Exploring the Lower Limits of Perfection

A fascinating dialectic has emerged between the observation of ``perfect liquid'' behavior by the RHIC sQGP [1], and the near-simultaneous conjecture of a fundamental bound on the ratio of viscosity to entropy density $\eta/s \geq 1/4\pi$ obtained by Kovtun, Son and Starinets (KSS) [2] via the AdS/CFT correspondence. While the existence of such a bound was anticipated by Danielewicz and Gyulassy based on simple quantum mechanical arguments [3], the possible connection to those conformal field theories with gravity duals studied by KSS and others makes the determination of the value of $\eta/s$ for the RHIC fluid particularly intriguing. This talk will consider various approaches utilizing flow, fluctuations, heavy quark transport and detailed second-order causal hydrodynamic simulations. The self- consistency, or lack thereof, of the resulting values for $\eta/s$ will be discussed, along with prospects for future improved measurements and theoretical analysis.\\ \parindent=0pt \textbf{References}\\ \parindent=0pt [1] http://www.bnl.gov/bnlweb/pubaf/pr/PR\_display.asp?prID=05- 38 \\ \parindent=0pt [2] P.~Kovtun, D.T.~Son and A.O.~Starinets, Phys.~Rev.~Lett. {\bf 94}, 111601 (2005) [arXiv:hep-th/0405231]. \\ \parindent=0pt [3] P.~Danielewicz and M.~Gyulassy, Phys.~Rev.~ {\bf D31}, 53 (1985).