Spin-axion coupling

TL;DR

This paper develops a covariant model describing how pseudoscalar (axion) fields influence the spin dynamics of test particles, including direct and indirect coupling mechanisms, with applications to cosmological, astrophysical, and gravitational wave scenarios.

Contribution

It introduces a new phenomenological covariant model for spin-axion interactions, encompassing both direct gradient coupling and electromagnetic mediation, with detailed solutions for various axion field configurations.

Findings

Spin precession occurs in inhomogeneous or non-stationary axion fields.

Exact solutions for spin dynamics in different axion backgrounds are obtained.

The model predicts observable spin rotations induced by relic, astrophysical, and gravitational wave axions.

Abstract

We establish a new covariant phenomenological model, which describes an influence of pseudoscalar (axion) field on spins of test massive particles. The model includes general relativistic equations of particle motion and spin evolution in background pseudoscalar (axion), electromagnetic and gravitational fields. It describes both the direct spin-axion coupling of the gradient type and indirect spin-axion interaction mediated by electromagnetic fields. Special attention is paid to the direct spin-axion coupling caused by the gradient of the pseudoscalar (axion) field. We show that it describes a spin precession, when the pseudoscalar (axion) field is inhomogeneous and/or non-stationary. Applications of the model, which correspond to the three types of four-vectors attributed to the gradient of the pseudoscalar (axion) field (time-like, space-like, and null), are considered in detail. These are the spin precessions induced by relic cosmological axions, axions distributed around spherically symmetric static objects, and axions in a gravitational wave field, respectively. We discuss features of the obtained exact solutions and some general properties of the axionically induced spin rotation.