Virtual Journal on QCD Matter

In April of 2005 Brookhaven National Laboratory announced that scientists at RHIC had created the most ideal liquid ever observed in nature. Now, 3 years later, RHIC is facing competition to this claim from strongly interacting ultra-cold Fermi gases.

The RHIC announcement, which has been covered widely in the international press (see e.g. articles in the NY Times and the BBC), refers to the remarkable success that ideal fluid dynamics has had in describing the elliptic flow and bulk properties exhibited by the matter created at RHIC. The theoretical consensus leading to this announcement was established during a RBRC workshop in May of 2004, the proceedings of which were published in Nucl. Phys. A, Vol. 750, Issue #1. The experimental data which served as the basis for these findings, including a detailed assessment by the experimental collaborations, were subsequently published in a second special volume of Nuclear Physics A:

• Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration
• Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions
• The PHOBOS perspective on discoveries at RHIC
• Quark gluon plasma and color glass condensate at RHIC? The Perspective from the BRAHMS experiment

In general, the success of ideal fluid dynamics, with the strong elliptic flow as its key feature, has been equated to the creation of strongly interacting matter. The reasoning is that ideal implies a vanishing viscosity, which in turn can be linked to large cross sections when utilizing kinetic theory and a purely particle based interpretation of matter. One should note, however, that this does not necessarily need to be the case, since hydrodynamics does not care about the origin of the viscosity, and field degrees of freedom may induce an anomalous viscosity without the need for large scattering cross sections.

Returning to strongly interacting matter, the well-known figure on the left has been widely used to demonstrate the existence of elliptic flow in other strongly interacting systems, namely an ultra-cold Fermi gas released from an optical trap. As a matter of fact, it has become so widely distributed in the RHIC community that most people have forgotten to give credit to the original reference (abstract, PDF) of this work published in Science by J.E. Thomas and his group at Duke University.

It turns out that our AMO friends have gotten quite intruiged by the KSS bound for the viscosity over entropy-density ratio (see also our post on the String Theory Connection), so now the competition is on who is able to claim the most ideal fluid in nature: RHIC or an optically trapped gas of Li atoms! Just recently Thomas’ group managed to perform an entropy measurement and made the observation of a near vanishing viscosity via the irrotational flow of a strongly interacting Fermi gas, which in turn allowed the group to come up with a first estimate of the viscosity/entropy-density ratio for their system: they find values for η/s as low as 0.4 . Even though this estimate is arguably still significantly higher than the KSS bound of 1/4π and the values needed to understand the RHIC data, the author of this post is willing to bet a bottle of Veuve Clicquot Ponsardin Brut (750 ML, no less) that it will decrease significantly in future measurements and prove tough competition for RHIC.

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