The ALP miracle revisited

Ryuji Daido,Wen Yin,Fuminobu Takahashi

doi:10.1007/jhep02(2018)104

Abstract

We revisit the ALP miracle scenario where the inflaton and dark matter are unified by a single axion-like particle (ALP). We first extend our previous analysis on the inflaton dynamics to identify the whole viable parameter space consistent with the CMB observation. Then, we evaluate the relic density of the ALP dark matter by incorporating uncertainties of the model-dependent couplings to the weak gauge bosons as well as the dissipation effect. The preferred ranges of the ALP mass and coupling to photons are found to be 0.01 ≲ mϕ ≲ 1 eV and {g}_{phi gamma gamma }=mathcal{O}left(1{0}^{-11}right) GeV−1, which slightly depend on these uncertainties. Interestingly, the preferred regions are within reach of future solar axion helioscope experiments, IAXO and TASTE, and laser-based stimulated photon-photon collider experiments. We also discuss possible extensions of the ALP miracle scenario by introducing interactions of the ALP with fermions.

Highlights

JHEP02(2018)104 which would necessitate a super-Planckian decay constant [15, 16]
The relic axion-like particle (ALP) condensate can explain the observed dark matter abundance if mφ = O(0.01–0.1) eV and gφγγ = O(10−11) GeV−1, which is within the reach of the future solar axion helioscope experiments, IAXO [22, 23] and TASTE [24], and laser-based photon colliders [25,26,27]
We show the regions constrained by the CAST experiment [47], the cooling argument of the horizontal branch (HB) stars [48] and the optical telescopes [49], and the projected sensitivity reach of the future experiments

Summary

The ALP inflation

Let us first explain the inflation model where the ALP plays the role of inflaton. We assume that the ALP enjoys a discrete shift symmetry, φ → φ + 2πf , where f is the decay constant. Note that the ALP mass mφ is of order the Hubble parameter during inflation This is because of the observational constraint on the spectral index. We stress that the relation (2.21) holds for a broader class of the ALP inflation (with e.g. more cosine terms) satisfying (2.2), as long as slow-roll inflation takes place in the vicinity of the potential maximum. This is partly because V (φ∗) is tightly constrained by the observation as it contributes to the running as well as the running of the running of the spectral index. Our argument on the reheating and the relic ALP abundance in the rest of this paper relies on the two relations (2.21) and λ ∼ O(10−12), and so, we expect that our results are not significantly modified for a broader class of the ALP inflation model

The ALP miracle

Coupling to photons and dissipation rates

Reheating through couplings to fermions

Thermalized ALPs as hot dark matter

Discussion and conclusions