Thermodynamic versus kinetic approach to polarization-vorticity coupling

TL;DR

This paper compares thermodynamic and kinetic methods for studying spin polarization and vorticity in particle systems, highlighting limitations of kinetic approaches and proposing a hydrodynamic framework incorporating spin.

Contribution

It clarifies the relationship between thermodynamic and kinetic approaches and introduces a hydrodynamic formulation for particles with spin, including pseudo-gauge transformations.

Findings

Thermodynamic approach shows spin polarization equals thermal vorticity at equilibrium.

Kinetic approach with vanishing collision term cannot relate vorticity and polarization.

Hydrodynamic equations for spinning particles are formulated with different energy-momentum tensors.

Abstract

We critically compare thermodynamic and kinetic approaches, that have been recently used to study relations between the spin polarization and fluid vorticity in systems consisting of spin-one-half particles. The thermodynamic approach refers to general properties of global thermal equilibrium with a rigid-like rotation and demonstrates that the spin-polarization and thermal-vorticity tensors are equal. On the other hand, the kinetic approach uses the concept of the Wigner function and its semi-classical expansion. In most of the works done so far, the Wigner functions satisfy kinetic equations with a vanishing collision term. We show that this assumption restricts significantly applicability of such frameworks and, in contrast to many claims found in the literature, does not allow for drawing any conclusions regarding the relation between the thermal-vorticity and spin-polarization tensors, except for the fact that the two should be constant in global equilibrium. We further show how the kinetic-theory equations including spin degrees of freedom can be used to formulate a hydrodynamic framework for particles with spin. We define hydrodynamic equations starting separately from the formulation by de~Groot, van~Leeuwen, and van~Weert and from the canonical formalism. In the former case the energy-momentum tensor is symmetric and the spin tensor is conserved, while in the later case the energy-momentum tensor is not symmetric and the spin tensor is not conserved. Nevertheless, in the two cases the total angular momentum is always conserved. Interestingly, the two approaches are connected by the pseudo-gauge transformation, which we explicitly define.