Observation of conformal symmetry breaking and scale invariance in expanding Fermi gases

Abstract

We precisely test scale invariance and local thermal equilibrium in the hydrodynamic expansion of a Fermi gas of atoms as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $⟨{\mathbf{r}}^2⟩=⟨x^2+y^2+z^2⟩$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.005(0.016)\hslash n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic "elliptic" flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $⟨{\mathbf{r}}^2⟩$ deviates from ballistic flow.

11 pages, 8 figures