title: Observation of conformal symmetry breaking and scale invariance in expanding Fermi gases.
abstract: We precisely test scale invariance and examine 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 <r2>=<x2+y2+z2> expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, 0.00(0.04)ℏn, 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 <r2> deviates from ballistic flow.
authors.affiliation: Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA and Department of Physics, Duke University, Durham, North Carolina 27708, USA.
authors.forename: E
authors.initials: E
authors.lastname: Elliott
authors.affiliation: Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Joseph
authors.affiliation: Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Thomas