http://arxiv.org/abs/1912.08188
An axion-like particle (ALP) with mass $m_\phi \sim 10^{-15}\text{ eV}$ oscillates with frequency $\sim$1 Hz and it appears naturally as a consequence of grand unification (GUT) in M-theory. However, with a GUT-scale decay constant such an ALP overcloses the Universe, and cannot solve the strong CP problem. In this paper, we present a two axion model in which the 1 Hz ALP constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density. The mechanism to achieve the correct relic densities relies on low-scale inflation ($m_\phi \lesssim H_{\rm inf}\lesssim 1 \text{ MeV}$) and stochastic drift of both axion fields, which also highly alleviates the cosmological moduli and gravitino problems. We present two variants of the model each compatible with cosmic microwave background anisotropies and big bang nucleosynthesis, one of which in addition satisfies the trans-Planckian censorship conjecture. The 1 Hz ALP has canonical GUT-scale couplings to both photons and nucleons, and may be detectable by ABRACADABRA and CASPEr. Furthermore the model predicts: (1) a value for the strong CP phase which oscillates around $\bar{\theta}_{\rm QCD}\sim 10^{-12}$, within reach of the proton storage ring electric dipole moment experiment, (2) that any intermediate mass black holes observed in gravitational waves from binary inspiral by LISA will have low dimensionless spin and reside on Regge trajectories.
D. Marsh and W. Yin
Wed, 18 Dec 19
68/71
Comments: 26 pages, 6 figures
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