Building far-from-equilibrium effective field theories using shift symmetries

TL;DR

This paper develops a unified framework for understanding far-from-equilibrium quantum field theories using shift symmetries, enabling interpolation between weak and strong coupling regimes and including stochastic effects.

Contribution

It introduces a novel effective field theory approach based on shift symmetries to describe nonhydrodynamic excitations far from equilibrium.

Findings

Constructed a far-from-equilibrium effective action incorporating shift symmetry.

Demonstrated the approach can interpolate between weak and strong coupling behaviors.

Analyzed a model relevant to thermalization in nuclear collisions considering QCD effects.

Abstract

Contemporary understanding of thermalization in quantum field theory stems largely from understanding properties of transient excitations of equilibria. These nonhydrodynamic excitations are known to structurally differ between weakly- and strongly-coupled quantum field theories with no known results at intermediate values of the interaction strength. We demonstrate that all the known behaviors of transient excitations can be understood as a consequence of different realizations of a symmetry principle, the shift symmetry, applied at the level of the far from equilibrium generalization of the hydrodynamic effective action that we explicitly construct. Our approach naturally includes the effects of stochastic fluctuations outside the hydrodynamic regime and allows to explicitly construct hybrid models interpolating between weak- and strong-coupling behavior. We study properties of one such model motivated by thermalization in nuclear collisions in light of the QCD running coupling.