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. We show that the bulk viscosity $\zeta$ scales as the second power of this parameter, $\zeta \sim (\frac{ z\lambda}{ a})^2 \lambda^{ -3}$.
Categories: cond-mat.quant-gas hep-ph nucl-th