Audible Axions with a Booster: Stochastic Gravitational Waves from Rotating ALPs

TL;DR

This paper explores how kinetic misalignment of axions can produce detectable stochastic gravitational waves at lower decay constants, expanding the potential for gravitational wave probes of axion-like particles and dark photon interactions.

Contribution

It introduces a new mechanism for gravitational wave generation from rotating axions with large initial velocities, including analytic estimates and numerical simulations, and assesses detectability and parameter constraints.

Findings

Detectable gravitational wave signals at lower decay constants.

Projected constraints on axion and dark photon parameter space.

Identification of regions where axions can be dark matter or generate baryon asymmetry.

Abstract

Gravitational waves provide a novel way to probe axions or axion-like particles coupled to a dark photon field, even in the absence of couplings to Standard Model particles. In the conventional misalignment mechanism, the generation of an observable stochastic gravitational wave background, however, requires large axion decay constants. We here investigate the gravitational wave signal generated within the kinetic misalignment scenario, where the axion is assumed to have a large initial velocity. Its kinetic energy then provides a sufficiently high energy budget to generate a detectable gravitational wave signal also at lower values of the decay constant. We obtain an analytic estimate as well as perform numerical simulations of the corresponding gravitational wave signal, and evaluate its detectability at current and future gravitational wave observatories. We further present the corresponding projected constraints on the parameter space of the model, along with the parameter regions in which the dark photon or axion constitute dark matter, or in which the baryon asymmetry of the Universe is generated via the axiogenesis mechanism. Finally we compute the GW production from the fragmentation of rotating axions, which is however difficult to observe experimentally.