Axion dark matter from first-order phase transition, and very high energy photons from GRB 221009A

Abstract

We study an axion-like particle (ALP) that experiences a first-order phase transition with respect to its mass or potential minimum. This can be realized if the ALP obtains a potential from non-perturbative effects of SU(N) gauge theory that is confined via the first-order phase transition. Similar dynamics are achieved in the so-called trapped misalignment mechanism, where the ALP is trapped in a false vacuum at high temperatures until it begins to oscillate about the true minimum. The resulting ALP abundance is significantly enhanced compared to the standard misalignment mechanism, explaining dark matter in a broader parameter space that is accessible to experiments e.g. IAXO, ALPS-II, and DM-radio. Furthermore, the viable parameter space includes a region of the mass ma≃10−8−10−7eV and the ALP-photon coupling gaγγ≃10−11GeV−1 that can explain the recent observation of very high energy photons from GRB221009A via axion-photon oscillations. The parameter region suggests that the FOPT can generate gravitational waves that explain the NANOGrav hint. If the ALP in this region explains dark matter, then the ALP might have experienced a first-order phase transition. Finally we also discuss cosmological aspects of the dark sector that triggers the FOPT and propose a possible solution to the cooling problem of dark glueballs.