Axion Dark Matter Models Research Articles

JWST’s PEARLS: Mothra, a new kaiju star at z = 2.091 extremely magnified by MACS0416, and implications for dark matter models

We report the discovery of Mothra, an extremely magnified monster star, likely a binary system of two supergiant stars, in one of the strongly lensed galaxies behind the galaxy cluster MACS J0416.1−2403. Mothra is in a galaxy with spectroscopic redshift z = 2.091 in a portion of the galaxy that is parsecs away from the cluster caustic. The binary star is observed only on the side of the critical curve with negative parity but has been detectable for at least eight years, implying the presence of a small lensing perturber. Microlenses alone cannot explain the earlier observations of this object made with the Hubble Space Telescope. A larger perturber with a mass of at least 104 M⊙ offers a more satisfactory explanation. Based on the lack of perturbation on other nearby sources in the same arc, the maximum mass of the perturber is 2.5 × 106 M⊙, making this the smallest substructure constrained by lensing at z > 0.3. The existence of this millilens is fully consistent with expectations from standard cold dark matter cosmology. On the other hand, the existence of such a small substructure in a cluster environment has implications for other dark matter models. In particular, warm dark matter models with particle masses below 8.7 keV are excluded by our observations. Similarly, axion dark matter models are consistent with the observations only if the axion mass is in the range 0.5 × 10−22 eV < ma < 5 × 10−22 eV.

The flavor of QCD axion dark matter

We argue that demanding a consistent cosmological history, including the absence of domain walls and strongly interacting relics at the Peccei-Quinn scale, singles out two concrete realizations of hadronic QCD axions as viable dark matter models. These realizations generally feature flavor-violating axion couplings to Standard Model quarks that are unsuppressed at low energies. As a consequence, experiments looking for flavor-violating hadronic processes involving the axion can be sensitive probes of QCD axion dark matter models. In particular, we show that the NA62 and KOTO experiments could detect the K → π + a decay for axions consistent with the observed dark matter abundance via the post-inflationary misalignment mechanism.

Resurrecting low-mass axion dark matter via a dynamical QCD scale

In the framework where the strong coupling is dynamical, the QCD sector may confine at a much higher temperature than it would in the Standard Model, and the temperature-dependent mass of the QCD axion evolves in a non-trivial way. We find that, depending on the evolution of ΛQCD, the axion field may undergo multiple distinct phases of damping and oscillation leading generically to a suppression of its relic abundance. Such a suppression could therefore open up a wide range of parameter space, resurrecting in particular axion dark-matter models with a large Peccei-Quinn scale fa ≫ 1012 GeV, i.e., with a lighter mass than the standard QCD axion.

Early-Universe Simulations of the Cosmological Axion.

Ultracompact dark matter (DM) minihalos at masses at and below 10^{-12} M_{⊙} arise in axion DM models where the Peccei-Quinn (PQ) symmetry is broken after inflation. The minihalos arise from density perturbations that are generated from the nontrivial axion self-interactions during and shortly after the collapse of the axion-string and domain-wall network. We perform high-resolution simulations of this scenario starting at the epoch before the PQ phase transition and ending at matter-radiation equality. We characterize the spectrum of primordial perturbations that are generated and comment on implications for efforts to detect axion DM. We also measure the DM density at different simulated masses and argue that the correct DM density is obtained for m_{a}=25.2±11.0 μeV.

Note on a solution to domain wall problem with the Lazarides-Shafi mechanism in axion dark matter models

Axion is a promising candidate of dark matter. After the Peccei-Quinn symmetry breaking, axion strings are formed and attached by domain walls when the temperature of the universe becomes comparable to the QCD scale. Such objects can cause cosmological disasters if they are long-lived. As a solution for it, the Lazarides-Shafi mechanism is often discussed through introduction of a new non-Abelian (gauge) symmetry. We study this mechanism in detail and show configuration of strings and walls. Even if Abelian axion strings with a domain wall number greater than one are formed in the early universe, each of them is split into multiple Alice axion strings due to a repulsive force between the Alice strings even without domain wall. When domain walls are formed as the universe cools down, a single Alice string can be attached by a single wall because a vacuum is connected by a non-Abelian rotation without changing energy. Even if an Abelian axion string attached by domain walls are created due to the Kibble Zurek mechanism at the chiral phase transition, such strings are also similarly split into multiple Alice strings attached by walls in the presence of the domain wall tension. Such walls do not form stable networks since they collapse by the tension of the walls, emitting axions.

Linking scalar dark matter and neutrino masses with IceCube 170922A

Two of the key unresolved issues facing Standard Model physics are (i) the appearance of a small but non-zero neutrino mass, and, (ii) the missing mass problem in the Universe. The focus of this paper is a previously proposed low energy effective theory that couples a dark scalar to Standard Model neutrinos. This provides a stable dark matter candidate as well as radiatively generating a neutrino mass. Within this framework we will then construct an entirely new bound from the IceCube-170922A event which takes into account (i) the possible neutrino mass hierarchies, (ii) the effect of cosmological redshift on e.g. the number density of cosmic neutrino background neutrinos, and, (iii) the non-degeneracy of neutrino mass and flavour eigenstates. This builds on work by Kelly and Machado (2018), where the authors placed new constraints on neutrinophilic and axion dark matter models. At low mediator masses, we find an improvement of an order of magnitude on current constraints from kaon decays. The constraint is complimentary (and slightly weaker) than current constraints from Big Bang Nucleosynthesis and the Cosmic Microwave Background. We explore how future higher energy events could improve this bound.

Exploring axionlike particles beyond the canonical setup

Axionlike particles (ALPs) are interesting dark matter candidates from both the theoretical and the experimental perspective. Usually they are motivated as pseudo-Nambu-Goldstone bosons. In this case one of their most important features is that their coupling to other particles is suppressed by a large scale, the vacuum expectation value of the field breaking the symmetry that gives rise to them. This naturally endows them with very weak interactions but also restricts the maximal field value and therefore the regions where sufficient dark matter is produced. In this paper we investigate deviations from this simplest setup, where the potential and interactions are as expected for a pseudo-Nambu-Goldstone boson, but the kinetic term has singularities. This leads to a significantly increased area in parameter space where such particles can be dark matter and can be probed by current and near future experiments. We discuss cosmological limits and in the course of this give a simple derivation of a formula for isocurvature fluctuations in models with general anharmonic potentials. As an application of this formula we give an update of the isocurvature constraints for QCD axion dark matter models, using the most recent results for the QCD topological susceptibility and the newest Planck data.

Fluctuations of the gravitational field generated by a random population of extended substructures

A large population of extended substructures generates a stochastic gravitational field that is fully specified by the function |$p(\boldsymbol {F})$|⁠, which defines the probability that a tracer particle experiences a force |$\boldsymbol {F}$| within the interval |${\boldsymbol F}, \boldsymbol {F}+{\rm d}{\boldsymbol F}$|⁠. This paper presents a statistical technique for deriving the spectrum of random fluctuations directly from the number density of substructures with known mass and size functions. Application to the subhalo population found in cold dark matter simulations of Milky Way sized haloes shows that, while the combined force distribution is governed by the most massive satellites, the fluctuations of the tidal field are completely dominated by the smallest and most abundant subhaloes. In light of this result, we discuss observational experiments that may be sufficiently sensitive to Galactic tidal fluctuations to probe the ‘dark’ low end of the subhalo mass function and constrain the particle mass of warm and ultralight axion dark matter models.

Tidal streams from axion miniclusters and direct axion searches

In some axion dark matter models a dominant fraction of axions resides in dense small-scale substructures, axion miniclusters. A fraction of these substructures is disrupted and forms tidal streams where the axion density may still be an order of magnitude larger than the average. We discuss implications of these streams for the direct axion searches. We estimate the fraction of disrupted miniclusters and the parameters of the resulting streams, and find that stream-crossing events would occur at a rate of about 1/(20 yr) for 2–3 days, during which the signal in axion detectors would be amplified by a factor ∼ 10. These estimates suggest that the effect of the tidal disruption of axion miniclusters may be important for direct axion searches and deserves a more thorough study.