A multipole-expanded effective field theory for vortex ring-sound interactions

TL;DR

This paper develops a multipole expansion method to simplify the effective field theory describing vortex ring interactions with sound in superfluids, enabling easier analysis of their dynamics and emitted sound.

Contribution

It introduces a systematic multipole expansion approach that recasts the complex two-form description into a simpler scalar field framework for vortex rings.

Findings

Derived a multipole-expanded effective action for vortex rings

Calculated sound emission from oscillating vortex rings

Simplified the description of vortex-sound interactions in superfluids

Abstract

The low-energy dynamics of a zero temperature superfluid or of the compressional modes of an ordinary fluid can be described by a simple effective theory for a scalar field---the superfluid 'phase'. However, when vortex lines are present, to describe all interactions in a local fashion one has to switch to a magnetic-type dual two-form description, which comes with six degrees of freedom (in place of one) and an associated gauge redundancy, and is thus considerably more complicated. Here we show that, in the case of vortex $rings$ and for bulk modes that are much longer than the typical ring size, one can perform a systematic multipole expansion of the effective action and recast it into the simpler scalar field language. In a sense, in the presence of vortex rings the non-single valuedness of the scalar can be hidden inside the rings, and thus out of the reach of the multipole expansion. As an application of our techniques, we compute by standard effective field theory methods the sound emitted by an oscillating vortex ring.