Abstract
An axion-like particle (ALP) with mass mϕ ∼ 10−15 eV oscillates with frequency ∼1 Hz. This mass scale lies in an open window of astrophysical constraints, and appears naturally as a consequence of grand unification (GUT) in string/M-theory. However, with a GUT-scale decay constant such an ALP overcloses the Universe, and cannot solve the strong CP problem. In this paper, we present a two axion model in which the 1 Hz ALP constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density. The mechanism to achieve the correct relic densities relies on low-scale inflation (mϕ ≲ Hinf ≲ 1 MeV), and we present explicit realisations of such a model. The scale in the axion potential leading to the 1 Hz axion generates a value for the strong CP phase which oscillates around {overline{theta}}_{mathrm{QCD}}sim {10}^{-12} , within reach of the proton storage ring electric dipole moment experiment. The 1 Hz axion is also in reach of near future laboratory and astrophysical searches.
Highlights
It is, on the other hand, known that the existence of an axion may lead to the accelerated growth of BHs [18]
We present a two axion model in which the 1 Hz axion-like particle (ALP) constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density
We have studied a two axion model inspired by M-theory with two would-be 1 Hz axions
Summary
We consider the model with two would-be 1 Hz axions, φ and a, each obtaining a potential of order Λvis, one of which will be later identified as the QCD axion.4 As we will discuss in the last section, the inclusion of more than two axions in the 1 Hz axion window does not change our conclusion.
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