Bulk viscosity and conformal symmetry breaking in the dilute Fermi gas near unitarity

Understand

The dilute Fermi gas at unitarity is scale invariant and its bulk viscosity vanishes.

• We compute the leading contribution to the bulk viscosity when the scattering length is not infinite.
• A measure of scale breaking is provided by the ratio $(P-\frac{2}{3}{\cal E})/P$, where $P$ is the pressure and ${\cal E}$ is the energy density.
• In the high temperature limit this ratio scales as $\frac{z\lambda}{a}$, where $z$ is the fugacity, $\lambda$ is the thermal wave length, and $a$ is the scattering length.