Axion dark matter from topological defects

Abstract

The cosmological scenario where the Peccei-Quinn symmetry is broken after inflation is investigated. In this scenario, topological defects such as strings and domain walls produce a large number of axions, which contribute to the cold dark matter of the universe. The previous estimations of the cold dark matter abundance are updated and refined based on the field-theoretic simulations with improved grid sizes. The possible uncertainties originated in the numerical calculations are also discussed. It is found that axions can be responsible for the cold dark matter in the mass range ma = (0.8-1.3) × 10 −4 eV for the models with the domain wall number NDW = 1, and ma ≈ O(10 −4 -10 −2 )eV with a mild tuning of parameters for the models with NDW > 1. Such higher mass ranges can be probed in future experimental studies.

Nonthermal axion dark radiation and constraints

“…We stress here that the restoration of the PQ symmetry and the subsequent formation of topological defects are only dangerous if N DW > 1. For models with N DW ¼ 1, however, these topological defects are unstable and decay into cold axions rendering the DM bound even stronger, f a ≲ ð4.6-7.2Þ × 10 10 GeV [18,19]. For the case of ϕ being the inflaton, we show the isocurvature bound on f a for θ i ¼ 1; 0.1; 0.01 and 0.001 in Figs.…”

“…Axions can be produced thermally if T rh > T a;dec , where T a;dec is given by Eq. (19). In particular, axions with decay constant f a ≲ Oð10 7 Þ GeV can be produced thermally and decouple from the plasma after the QCD phase transition.…”

“…It is important to note here that for sufficiently large f a axions are never in thermal contact with the plasma and hence keep their initial abundance and momenta, see Eq. (19). Therefore, such axions are most likely to act as dark radiation contributing to N eff .…”

“…On the other hand, when N DW > 1 (i.e. for DFSZ-like models or KSVZ-like models with several extra heavy quark species), the topological defects are stable and dominate the energy density of the Universe ruling out such scenario unless one fine-tunes a bias term that explicitly breaks the shift symmetry, and in this case f PQ is constrained to be less than Oð10 10 Þ GeV in order to avoid the overproduction of axions [19,20]. 3 Here we consider a negative coupling to ϕ, i.e.…”

“…(1), gϕ 2 jSj 2 , will 2 For N DW ¼ 1 (i.e. for KSVZ-like models with only one extra heavy quark species), these defects are unstable and decay to cold axions leading to an upper bound on the PQ breaking scale, f PQ ≲ ð4.6-7.2Þ × 10 10 GeVðΩ a =Ω CDM Þ 0.84 , where Ω a denotes the cold axions abundance and Ω CDM is the observed abundance of cold dark matter (CDM) [18,19]. On the other hand, when N DW > 1 (i.e.…”