Schwinger-Keldysh effective theory of charge transport: redundancies and systematic $\omega/T$ expansion

TL;DR

This paper develops a comprehensive effective field theory framework for non-Abelian charge transport near thermal equilibrium, unifying different approaches and extending to all orders in frequency over temperature.

Contribution

It demonstrates the equivalence of two existing formalisms, extends them to include DKMS symmetry at all orders, and clarifies power-counting rules for semiclassical and hydrodynamic expansions.

Findings

Proved the equivalence of Goldstone and adjoint matter field formalisms.

Extended formalisms to be compatible with DKMS symmetry at all orders.

Provided power-counting rules for semiclassical and hydrodynamic expansions.

Abstract

We study Schwinger-Keldysh effective field theories (EFTs) for systems with non-Abelian internal symmetries near thermal equilibrium. We consider two approaches that were put forward in the literature -- one using a redundant Goldstone parameterization, the other employing an adjoint matter field -- and demonstrate their complete equivalence by providing an explicit dictionary and proving their equivalence at the path integral level. Critically, we extend both formalisms to be compatible with the dynamical Kubo-Martin-Schwinger (DKMS) symmetry to all orders in $\hbar \omega /T$, classifying all possible invariant kernels satisfying unitarity constraints. We also establish precise power-counting rules, clarifying the interplay between the semiclassical and hydrodynamic expansions. Our work provides a framework for studying non-Abelian charge transport and fluctuations to arbitrary orders in $\hbar \omega /T$.