Gauge transformations and Galilean covariance in nonlinear gauge-coupled quantum fluids

We investigate certain invariance properties of quantum fluids subject to a nonlinear gauge potential. In particular, we derive the covariant transformation laws for the nonlinear potentials under a space-time Galilean boost and consider U(1) gauge transformations. We find that the hydrodynamic canonical field equations are form-invariant in the case of external gauge functions χ(r,t), but not for nonlinear gauge functionals χ[ρ]. Hence, nonlinear gauge potentials are nontrivial potentials which may not be "gauged-away."Notably, for a superfluid in dimension d=1, attempting to do so generates the gauge pressure of the fluid in the Hamiltonian density. Furthermore, we investigate how the field equations transform under arbitrary Galilean transformations. We find that the immediate lack of Galilean covariance is restored under a suitably chosen transformation rule set for the potentials, which is identical in form to that of a Schrödinger particle coupled to external scalar and vector potentials.