Velocity operator approach to quantum fluid dynamics in a   three-dimensional neutron-proton system
  In the preceeding paper, introducing isospin-dependent density operators and defining exact momenta (collective variables), we could get an exact canonically momenta approach to a one-dimensional (1D) neutron-proton (NP) system. In this paper, we attempt at a velocity operator approach to a 3D NP system. Following Sunakawa, after introducing momentum density operators, we define velocity operators, denoting classical fluid velocities. We derive a collective Hamiltonian in terms of the collective variables. 
Seiya Nishiyama and Joao da Providencia
nucl-th