The phenomenology of Axion Relic Pockets

Abstract

Axion relic pockets are phase-transition remnants consisting of regions of false vacuum stabilised from collapse by a hot axion gas. Axion relic pockets can comprise dark matter, but little is known about their phenomenology. We consider trapped axions coupled to electromagnetism and derive the axion to photon conversion rate accounting for the pocket's compactness and spherical geometry. As a case study, we show that axion-photon conversion occurs in atomic electric fields, leading to high-energy electromagnetic cascades that may be detectable by neutrino, cosmic-ray, and dark-matter experiments. The distribution of electromagnetic showers is inherently isotropic, and upward-going showers provide a smoking-gun signal for the model. Using recent data from the IceCube neutrino telescope, we derive the first limits from terrestrial experiments on axion relic pockets as dark matter. However, for the simplest realisation of the model, only a small part of the parameter space can give signals strong enough to be constrained by current data. We also comment on the prospects for discovering axion relic pockets using fluorescence and radio Cherenkov detection by atmospheric instruments and balloon-borne payloads, as well as traditional dark matter direct detection experiments. These results provide the necessary tools to initiate phenomenological studies of axion relic pockets across different environments.