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We precisely test scale invariance and local thermal equilibrium in thehydrodynamic expansion of a Fermi gas of atoms as a function of interactionstrength. After release from an anisotropic optical trap, we observe that aresonantly interacting gas obeys scale-invariant hydrodynamics, where the meansquare cloud size $\langle{\mathbf{r}}^2\rangle=\langle x^2+y^2+z^2\rangle$expands ballistically (like a noninteracting gas) and the energy-averaged bulkviscosity is consistent with zero, $0.005(0.016)\,\hbar\,n$, with $n$ thedensity. In contrast, the aspect ratios of the cloud exhibit anisotropic"elliptic" flow with an energy-dependent shear viscosity. Tuning away fromresonance, we observe conformal symmetry breaking, where$\langle{\mathbf{r}}^2\rangle$ deviates from ballistic flow.

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