Cosmic Axion Force

TL;DR

This paper explores how extremely light axions can mediate long-range CP-violating forces between dark matter and visible matter, leading to detectable spin precession effects and providing new experimental avenues for axion detection.

Contribution

It introduces the concept of long-range forces mediated by ultralight axions with CP violation, and proposes experimental methods to detect these forces via spin precession measurements.

Findings

Long-range axion-mediated forces can cause spin precession detectable by magnetometers.

The effective magnetic field from axions could be observed if axion parameters satisfy certain mass and decay constant ranges.

Axionic structures like domain walls can also induce detectable magnetic effects.

Abstract

Nambu-Goldstone bosons, or axions, may be ubiquitous. Some of the axions may have small masses and thus serve as mediators of long-range forces. In this paper, we study the force mediated by an extremely light axion, $\phi$, between the visible sector and the dark sector, where dark matter lives. Since nature does not preserve the CP symmetry, the coupling between dark matter and $\phi$ is generically CP-violating. In this case, the induced force is extremely long-range and behaves as an effective magnetic field. If the force acts on electrons or nucleons, the spins of them on Earth precess around a fixed direction towards the galactic center. This provides an experimental opportunity for $\phi$ with mass, $m_\phi$, and decay constant, $f_\phi$, satisfying $m_\phi\lesssim 10^{-25}\,$ eV, $f_\phi\lesssim 10^{14}\,$GeV if the daily modulation of the effective magnetic field signals in magnetometers is measured by using the coherent averaging method. The effective magnetic field induced by an axionic compact object, such as an axion domain wall, is also discussed.