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It was recently conjectured that the ratio of the shear viscosity to entropydensity, $ \eta/ s$, for any fluid always exceeds $\hbar/(4 \pi k_B)$. Thisconjecture was motivated by quantum field theoretic results obtained via theAdS/CFT correspondence and from empirical data with real fluids. A theoreticalcounterexample to this bound can be constructed from a nonrelativistic gas byincreasing the number of species in the fluid while keeping the dynamicsessentially independent of the species type. The question of whether theunderlying structure of relativistic quantum field theory generically inhibitsthe realization of such a system and thereby preserves the possibility of auniversal bound is considered here. Using rather conservative assumptions, itis shown here that a metastable gas of heavy mesons in a particular controlledregime of QCD provides a realization of the counterexample and is consistentwith a well-defined underlying relativistic quantum field theory. Thus, quantumfield theory appears to impose no lower bound on $\eta/s$, at least formetastable fluids.Comment: 4 pages; typos corrected and references added in new versio

Is There a “Most Perfect Fluid” Consistent with Quantum Field Theory?