Session P6: Fermions at Unitarity: Gravity, the Quark-Gluon Plasma, and Ultra-Cold Atoms

Exploring perfect fluidity in universal atomic gases

Fermionic systems at unitarity exhibit universal behavior independent of the microscopic interactions. That such systems exist across vastly different fields of physics such as the quark-gluon plasma, neutron matter and ultracold atomic gases allows insights and studies in one to yield consequences to the others. One such common thread relates the hydrodynamics of the universal system to its thermodynamics through the ratio of its shear viscosity to its entropy density. By string theoretic methods, this ratio is currently believed to be bounded from below, in essence defining the perfect fermionic fluid. We study this state of matter in an ultracold atomic gas of $^6$Li. A mixture of atoms with spins $\pm$1/2 up/down is tuned with a magnetic field near a broad Feshbach resonance where the scattering cross section between the two spin states diverge. This, along with the ultracold temperatures, ensures a two-body scattering amplitude at the unitary limit. Our previous measurement of the entropy of this strongly-interacting system provides half of the information needed to test the string theory bound. The other half comes from new measurements of viscosity in the normal (non-superfluid) phase of the gas. Viscosity measurements are made by studying the evolution of the density of a rotating and expanding cloud of atoms. Comparison to ideal hydrodynamic motion allows for an estimate of the viscosity. We present the data from these studies and their preliminary analysis, including a comparison to the string theory limit.   

Quark Gluon Plasma and Fermions at Unitarity

The quark gluon plasma created in ultra-relativistic heavy ion collisions and dilute ultracold Fermi gases near unitarity are both strongly correlated quantum fluids that exhibit nearly perfect fluidity, that means the ratio of shear viscosity to entropy density approaches a lower bound that is believed to follow from the uncertainty relation, and that has been made more precise using calculations based on string theory. We will review these arguments and summarize recent efforts to extract transport properties of the quark gluon plasma and of ultra-cold Fermi gases.   

What do we know about the unitary Fermi gas?

A little more than ten years ago George Bertsch realized that the system of spin-1/2 fermions interacting with a zero range and infinite scattering is a nontrivial problem, in spite and also because of the absence of any dimensional scales, apart from the separation among particles. While it was clear at that time that this rather abstract problem has some relevance to the physics of neutron stars, soon it became evident that this is problem of much larger interest, in particular for cold fermionic atoms in traps. While there were many attempts over the years to elucidate the properties of this system using analytical methods, various approximate many-body techniques but so far solutions with a controlled accuracy failed to be produced. Quantum Monte Carlo (QMC) methods however have been particularly successful and they serve as the reference for experimentalists and theorists alike. With QMC methods applied either at zero or finite temperature a large number of properties of the Fermi gas in the unitary regime have been elucidated. This system proved to be a somewhat unique quantum superfluid: it has the largest critical temperature and pairing gap (in appropriate units) known; it has the largest critical velocity. It has been also possible to use QMC techniques in conjunction with Density Functional Theory to show that many other exotic phases are extremely likely to be realized in these systems: the Fulde-Ferrell-Larkin-Ovchinnikov and induced p-wave pairing. In the latest turn of events it was shown that this system demonstrates the existence of the pseudogap phenomenon as well, a property shared with high temperature superconductors.   

Study of the Perfect Liquid at RHIC

The Relativistic Heavy Ion Collider (RHIC) has been providing high energy collisions of nuclei (from protons to gold) since 2000, with the goal of studying strongly interacting matter at the highest densities and temperatures achieved in the laboratory. Ten years of RHIC data have revealed the system formed in these collisions are a ``perfect fluid,'' one with very low viscosity -- perhaps even saturating the vicosity bound predicted from string theory. This system has been labeled the ``strongly-coupled Quark-Gluon Plasma,'' or sQGP, to distinguish it from the weakly-coupled system expected at high energies expected previously as a consequence of asymptotic freedom. This talk will outline the experimental program at RHIC, and highlight several of the most important results leading to the current understanding of RHIC data. It will also explain how both the RHIC upgrades and the upcoming Pb+Pb program at the CERN LHC will contribute to our understanding of the microscopic processes that lead to the formation and evolution of this novel strongly-coupled medium.