  For two-dimensional non-dissipative fluids with broken parity, we show via
effective field theory methods that the infrared dynamics generically exhibit
Hall viscosity--a conservative form of viscosity compatible with
two-dimensional isotropy. The equality between the Hall viscosity coefficient
and the ground state's intrinsic angular momentum density follows
straightforwardly from their descending from the same Lagrangian term of the
low-energy effective action. We show that for such fluids sound waves are not
purely longitudinal, but acquire an elliptical polarization, with
transverse-to-longitudinal aspect ratio proportional to frequency. Our analysis
is fully relativistic, thus providing a natural description of (2+1)
dimensional relativistic fluids with broken parity.
