Gravitational Signatures of Axion Dark Matter via Parity-Violating Interactions

TL;DR

This work examines a parity-violating CS coupling between axion DM and gravity, which enables axions to decay into gravitons and generate a persistent, nearly monochromatic GW line. Using an NFW Galactic halo model, the authors compute the resulting graviton flux and translate non-detections into bounds on the coupling $\alpha$, comparing with LIGO O4 data and Einstein Telescope projections. They find that for $m_\phi \lesssim 10^{-11}$ eV, terrestrial GW searches can beat cosmological stability bounds by several orders of magnitude, providing the first robust direct constraints on the axion–gravity CS interaction. The study also outlines distinctive signatures, such as circular polarization and annual modulation, and discusses avenues for enhancement from DM substructures and broader frequency coverage with space-based detectors. Overall, GW interferometry emerges as a direct probe of parity-violating gravity and axion physics, with future detectors poised to extend sensitivity across wider mass ranges.

Abstract

We investigate axion-like particles coupled to gravity through a parity-violating Chern-Simons (CS) interaction. In this framework, axion dark matter (DM) can decay into pairs of circularly polarized gravitons, producing a persistent, nearly monochromatic GW signal. We compute the expected signal at Earth assuming a Navarro-Frenk-White Galactic halo model with the corresponding velocity distribution, and compare it with the narrowband sensitivities of the LIGO O4 run and the projected reach of the Einstein Telescope. The resulting bounds on the axion-graviton coupling $α$ improve upon the cosmological stability requirement for axion masses $m_φ\lesssim 10^{-11}$ eV, excluding values up to four orders of magnitude below the stability limit. This constitutes a robust direct terrestrial constraint on the axion-gravity CS coupling. We also discuss distinctive observational signatures, such as circular polarization asymmetries, annual modulation, and potential enhancements from DM substructures, which could serve as smoking-gun evidence for parity-violating gravitational interactions.

Figures (1)

• Figure 1: Axion-graviton coupling $\alpha$ as function of the axion mass $m_\phi$. The green solid line denotes the cosmological stability bound in Eq. \ref{['eq:stability']}. The red shaded region is excluded by the non-detection of a monochromatic signal in LIGO O4 data, and the blue shaded region shows the projected exclusion reach of ET. Ground-based interferometers improve upon the stability bound by several orders of magnitude for $10^{-14} \lesssim m_\phi/{\rm eV} \lesssim 10^{-11}$.