Zilch Vortical Effect, Berry Phase, and Kinetic Theory

TL;DR

This paper develops a covariant chiral kinetic theory for photons, linking zilch and helicity currents to Berry phase effects and quantum anomalies, advancing understanding of chiral transport in rotating photon gases.

Contribution

It constructs a first-principles kinetic theory for photons, revealing the role of Berry phase and gauge freedom in chiral transport phenomena.

Findings

Zilch and helicity currents originate from Berry phase effects.

The frame vector for photon spin is identified as residual gauge freedom.

Connections between vortical responses and quantum anomalies are discussed.

Abstract

Rotating photon gas exhibits a chirality separation along the angular velocity which is manifested through a generation of helicity and zilch currents. In this paper we study this system using the corresponding Wigner function and construct elements of the covariant chiral kinetic theory for photons from first principles. The Wigner function is solved order-by-order in $\hbar$ and the unconstrained terms are fixed by matching with quantum field theory results. We further consider the zilch and helicity currents and show that both manifestations of the chirality transport originate in the Berry phase of photons similarly to other chiral effects. Constructing the kinetic description from the Wigner function we find that the frame vector needed to fix the definition of spin of a massless particle is, in fact, the vector of the residual gauge freedom for the free Maxwell theory. We also briefly comment on the possible relation between vortical responses in rotating systems of massless particles and the anomalies of underlying quantum field theory.