Axion Dark Matter Research Articles

Baryogenesis from decaying magnetic helicity in axiogenesis

Generating axion dark matter through the kinetic misalignment mechanism implies the generation of large asymmetries for Standard Model fermions in the early universe. Even if these asymmetries are washed out at later times, they can trigger a chiral plasma instability in the early universe. Similarly, a direct coupling of the axion with the hypercharge gauge field can trigger a tachyonic instability. These instabilities produce helical magnetic fields, which are preserved until the electroweak phase transition. At the electroweak phase transition, these become a source of baryon asymmetry, which can be much more efficient than the original axiogenesis proposal. We discuss constraints on axion dark matter production from the overproduction of the baryon asymmetry as well as a minimal, albeit fine-tuned setup, where both the correct dark matter abundance and baryon asymmetry can be achieved. For a given axion decay constant, this leads to a sharp prediction for the mass of the radial direction of the Peccei Quinn field, which is a soft mass scale in supersymmetric theories.

QCD axion dark matter and the cosmic dipole problem

QCD axion dark matter and the cosmic dipole problem

On the use of dielectric elements in axion searches with microwave resonant cavities

This study explores the primary effects of dielectric materials in a resonant cavity-based search for axion dark matter. While dielectrics prove beneficial in numerous cases, their incorporation may lead to less-than-optimal performance, especially for the lowest TM mode. Additionally, the stronger confinement of the electric field inside the dielectrics can exacerbate mode mixings, in particular for higher-order modes. Case studies have been carried out using a combination of analytical solutions and numerical simulations. The findings indicate dielectric cavities employing the TM010 mode experience a significant reduction in sensitivity when compared to a similar search conducted in a cavity at equivalent frequency using no dielectrics.

Dark matter and dark radiation from the early universe with a modulus coupled to the PQMSSM

The supersymmetrized DFSZ axion model is especially compelling in that it contains 1. the SUSY solution to the gauge hierarchy problem, 2. the Peccei-Quinn (PQ) solution to the strong CP problem and 3. the Kim-Nilles solution to the SUSY μ problem. In a string setting, where a discrete R-symmetry ( {textbf{Z}}_{24}^R for example) may emerge from the compactification process, a high-quality accidental axion (accion) can emerge from the accidental, approximate remnant global U(1)PQ symmetry where the decay constant fa is linked to the SUSY breaking scale, and is within the cosmological sweet zone. In this setup, one also expects the presence of stringy remnant moduli fields ϕi. Here, we consider the situation of a single light modulus ϕ coupled to the PQMSSM in the early universe, with mixed axion plus higgsino-like WIMP dark matter. We evaluate dark matter and dark radiation production via nine coupled Boltzmann equations and assess the severity of the cosmological moduli problem (CMP) along with dark matter and dark radiation production rates. We find that typically the light modulus mass should be mϕ ≳ 104 TeV to avoid the moduli-induced dark matter overproduction problem. If one is able to (anthropically) tune the modulus field amplitude, we find a value of ϕ0 ≲ 10–7mP would be required to solve the overall CMP.

Probing dark matter axions using the hyperfine structure splitting of hydrogen atoms

QCD axions can be a substantial part of dark matter if their mass ma∼10−5 eV. Since the axions were created by the misalignment mechanism, their local energy spectrum density is large. Consequently, the axion-induced atomic transition rate is enhanced if the atomic energy gap matches the axion mass. The hyperfine splitting between the spin 0 singlet ground state and the spin 1 triplet state of hydrogen is 0.59×10−5 eV, which is close to the preferred mass of dark matter axions. With an energy gap adjustment by applying a weak Zeeman magnetic field, dark matter axions can induce atomic hydrogen transitions. Furthermore, because the total spins of the hydrogen triplet and singlet differ, the axion-induced transitions are detectable by a Stern–Gerlach apparatus or a sensitive magnetic field detector. A potential realization of the proposed scheme can be similar to existing hydrogen masers.

Opening up a Window on the Postinflationary QCD Axion.

The QCD axion cosmology depends crucially on whether the QCD axion is present during inflation or not. We point out that contrary to the standard criterion, the Peccei-Quinn (PQ) symmetry could remain unbroken during inflation, even when the axion decay constant, f_{a}, is (much) above the inflationary Hubble scale, H_{I}. This is achieved through the heavy lifting of the PQ scalar field due to its leading nonrenormalizable interaction with the inflaton, encoded in a high-dimensional operator which respects the approximate shift symmetry of the inflaton. The mechanism opens up a new window for the post-inflationary QCD axion and significantly enlarges the parameter space, in which the QCD axion dark matter with f_{a}>H_{I} could be compatible with high-scale inflation and free from constraints on axion isocurvature perturbations. There also exist nonderivative couplings, which still keep the inflaton shift symmetry breaking under control, to achieve the heavy lifting of the PQ field during inflation. Additionally, by introducing an early matter domination era, more parameter space of high f_{a} could yield the observed DM abundance.

Limits on Dark Photons, Scalars, and Axion‐Electromagnetodynamics with the ORGAN Experiment

AbstractAxions are a well‐motivated dark matter candidate, with a host of experiments around the world searching for direct evidence of their existence. The ORGAN Experiment is a type of axion detector known as an axion haloscope, which takes the form of a cryogenic resonant cavity embedded in a strong magnetic field. ORGAN recently completed Phase 1a, a scan for axions ≈65 µeV, and placed the most stringent limits to date on the dark matter axion–photon coupling in this region, . It has been shown that axion haloscopes such as ORGAN are automatically sensitive to other kinds of dark matter candidates, such as dark photons, scalar field/dilaton dark matter, and exotic axion–electromagnetic couplings motivated by quantum electromagnetodynamics. The exclusion limits placed on these various dark matter candidates are computed by ORGAN 1a, and sensitivity for some future ORGAN phases are projected. In particular, the dark photon limits are the most sensitive to date in some regions of the parameter space.

Naturally astrophobic QCD axion

We present a QCD axion model where the couplings of the axion to nucleons, electrons, and muons are naturally suppressed because of the appropriate choice of the Peccei-Quinn charges of the Standard Model fermions. We reexamine next-to-leading order corrections to the couplings of the axion with nucleons and photons and show that the axion decay constant may be as small as 107 GeV. It is also possible to suppress the coupling with the photon so that the decay constant is even smaller and minimal axiogenesis works. In this scenario, the axion has a mass above 1 eV and may be directly detected via absorption of axion dark matter. Flavor-violating axion couplings are generically predicted in our model, but we show that they may be naturally and sufficiently suppressed. We discuss the implications of the hints for anomalous cooling in several stellar environments to our model.

Early universe dynamics of PQ field with very small self-coupling and its implications for axion dark matter

Axion-like particles (ALPs) are often considered as good candidates for dark matter. Several mechanisms generating relic abundance of ALP dark matter have been proposed. They may involve processes which take place before, during or after cosmic inflation. In all cases an important role is played by the potential of the corresponding Peccei-Quinn (PQ) field. Quite often this potential is assumed to be dominated by a quartic term with a very small coupling. We show that in such situation it is crucial to take into account different kinds of corrections especially in models in which the PQ field evolves during and after inflation. We investigate how such evolution changes due to radiative, thermal and geometric corrections. In many cases those changes are very important and result in strong modifications of the predictions of a model. They may strongly influence the amount of ALP contributions to cold and warm components of dark matter as well as the power spectrum of associated isocurvature perturbations. Models with a quasi-supersymmetric spectrum of particles to which the PQ field couples seem to be especially interesting.Qualitative features of such models are discussed with the help of approximate analytical formulae. However, the dynamics of the PQ field with the considered corrections taken into account is more complicated than in the case without corrections so dedicated numerical calculations are necessary to obtain precise predictions.We present such results for some characteristic benchmark points in the parameter space.

Manifestation of Dark Matter Axions in Spin Effects in Storage Rings

Manifestation of Dark Matter Axions in Spin Effects in Storage Rings

Seeking dark matter with γ -ray attenuation

The flux of high-energy astrophysical $\ensuremath{\gamma}$ rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from $\ensuremath{\gamma}$-ray measurements exceeds that expected from galaxies at the $\ensuremath{\sim}2\ensuremath{\sigma}$ level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then an excess (an overall 14--30% increase of the EBL) is favored over the null hypothesis of no excess at the $2.7\ensuremath{\sigma}$ level. If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis at the $2.1\ensuremath{\sigma}$ confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the 8--25 eV mass range. This work highlights the sensitivity of $\ensuremath{\gamma}$-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations.

Thermal production of cold “hot dark matter” around eV

A very simple production mechanism of feebly interacting dark matter (DM) that rarely annihilates is thermal production, which predicts the DM mass around eV. This has been widely known as the hot DM scenario. Despite there are several observational hints from background lights suggesting a DM in this mass range, the hot DM scenario has been considered strongly in tension with the structure formation of our Universe because the free-streaming length of the DM produced from thermal reactions was thought to be too long. In this paper, I show that the previous conclusions are not always true depending on the reaction for bosonic DM because of the Bose-enhanced reaction at very low momentum. By utilizing a simple 1 ↔ 2 decay/inverse decay process to produce DM, I demonstrate that eV range bosonic DM can be thermally produced in a cold manner from a hot plasma through a model-independent analysis applicable to axion, hidden photon, and other bosonic DM candidates. As a result, bosonic DM in the eV mass range may still be unique and theoretically well-motivated. I also discuss some caveats arising from this phenomenon in the freeze-in production of DM, and present a related system that can suppress the hot plasma with thermal reaction.

Observational constraint on axion dark matter in a realistic halo profile with gravitational waves

Axions are considered as a candidate of dark matter. The axions form coherent clouds which delay and amplify gravitational waves (GWs) at a resonant frequency. That is, the coherent axion clouds produce secondary GWs following a primary wave. All GWs from compact binary mergers detected so far propagate in the Milky Way halo composed of dark matter so that such GWs are followed by the secondary GWs. Since the properties of the secondary GWs depend on the axion mass and coupling with the parity violating sector of gravity, we can search for a characteristic signal produced by axions. In our previous study, we have developed a search method optimized for the axion signals, and obtained about ten times stronger constraint on the coupling than the previous best one for the axion mass range, $[1.7 \times 10^{-13},\,8.5 \times 10^{-12}]\,\mathrm{eV}$. However, in our previous search, we assume that dark matter is homogeneously distributed in the Milky Way halo, which is unrealistic. In this paper, we extend the dark matter profile to a more realistic one called the core NFW profile, and find that the constraint in our previous study is robust to the dark matter profile.

Lepto-axiogenesis and the scale of supersymmetry

If the Peccei-Quinn field containing the QCD axion undergoes rotations in the early universe, the dimension-five operator responsible for neutrino masses can generate a lepton asymmetry that ultimately gives rise to the observed baryon asymmetry of the Universe. This lepto-axiogenesis scenario requires a flat potential for the radial direction of the Peccei-Quinn field, naturally realized in supersymmetric models. We carefully compute the efficiency of this mechanism for the Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) and Kim-Shifman-Vainshtein-Zakharov (KSVZ) axion models and place lower bounds on the masses of scalar superpartners required to reproduce the observed baryon asymmetry. For the KSVZ model, we find an efficiency for generation of the asymmetry six times larger than the previously extant computation after including scattering channels involving superpartners. In this case, the superpartner scale should be above 30 TeV for a domain wall number of one; the lower bound weakens for larger domain wall numbers. We find that the superpartner mass scale may also be as low as ~ 30 TeV for the DFSZ model. In all cases, the lower bound on the superpartner masses is inversely proportional to the sum of the squares of the neutrino masses and so can strengthen as the upper bound on the neutrino mass improves. We identify the parameter space where the axion rotation can simultaneously produce axion dark matter via kinetic misalignment; in this case it is possible to put an upper bound of order PeV on the masses of scalar superpartners.

Dark matter axions in the early universe with a period of increasing temperature

We consider the production of axion dark matter through the misalignment mechanism in the context of a nonstandard cosmological history involving early matter domination by a scalar field with a time-dependent decay rate. In cases where the temperature of the Universe experiences a temporary period of increase, Hubble friction can be restored in the evolution of the axion field, resulting in the possibility of up to three “crossings” of the axion mass and the Hubble expansion rate.This has the effect of dynamically resetting the misalignment mechanism to a new initial state for a second distinct phase of oscillation.The resultant axion mass required for the present dark matter relic density is never bigger than the standard-history window and can be smaller by more than three orders of magnitude, which can be probed by upcoming experiments such as ABRACADABRA, KLASH, ADMX, MADMAX, and ORGAN, targeting the axion-photon coupling. This highlights the possibility of exploring the cosmological history prior to Big Bang Nucleosynthesis through searches for axion dark matter beyond the standard window.

Heating up Peccei-Quinn scale

We discuss production of QCD axion dark matter in a novel scenario, which assumes time-varying scale of Peccei-Quinn symmetry breaking. The latter decreases as the Universe's temperature at early times and eventually stabilises at a large constant value. Such behavior is caused by the portal interaction between the complex field carrying Peccei-Quinn charge and a Higgs-like scalar, which is in thermal equilibrium with primordial plasma. In this scenario, axions are efficiently produced during the parametric resonance decay of thecomplex Peccei-Quinn field, relaxing to the minimum of its potential in the radiation-dominated stage. Notably, this process is not affected by the Universe's expansion rate and allows to generate the required abundance of dark matter independently of an axion mass. Phenomenological constraints on the model parameter space dependon the number density of radial field fluctuations, which are also generically excited along with axions, and the rate of their thermalization in the primordial plasma.For the ratio of radial field and axion particles number densities larger than ∼ 0.01 at the end of parametric resonance decay, the combination of cosmological and astrophysical observations with the CAST limit confinesthe Peccei-Quinn scale to a narrow range of values ∼ 108 GeV, —this paves the way for ruling out our scenario with the near future searches for axions.

An improved synthetic signal injection routine for the Haloscope At Yale Sensitive To Axion Cold dark matter (HAYSTAC).

Microwave cavity haloscopes are among the most sensitive direct detection experiments searching for dark matter axions via their coupling to photons. When the power of the expected microwave signal due to axion-photon conversion is on the order of 10-24 W, having the ability to validate the detector response and analysis procedure by injecting realistic synthetic axion signals becomes helpful. Here, we present a method based on frequency hopping spread spectrum for synthesizing axion signals in a microwave cavity haloscope experiment. It allows us to generate a narrow and asymmetric shape in frequency space that mimics an axion's spectral distribution, which is derived from a Maxwell-Boltzmann distribution. In addition, we show that the synthetic axion's power can be calibrated with reference to the system noise. Compared to the synthetic axion injection in the Haloscope At Yale Sensitive to Axion Cold dark matter (HAYSTAC) Phase I, we demonstrated synthetic signal injection with a more realistic line shape and calibrated power.

Simulation of classical axion electrodynamics using COMSOL multiphysics

The axion is a hypothetical particle motivated to address the strong CP problem, and is one of the appealing dark matter candidates. Numerous experimental searches for dark matter axions have been proposed relying on their coupling with photons. The classical equations of motion for the axion-photon coupling are well known but need to be fully computed for complex experimental setups. The partial differential equations of axion electrodynamics can be numerically solved using finite element methods. In this work, we simulate axion electrodynamics using COMSOL Multiphyics, a commercially available simulation software, for various experimental schemes, including the dish antenna haloscope, cavity haloscope, dielectric haloscope, and axion-photon regeneration. We show that the numerical results are in good agreement with the analytical solutions.

Axion minivoids and implications for direct detection

In the scenario in which QCD axion dark matter is produced after inflation, the Universe is populated by large inhomogeneities on very small scales. Eventually, these fluctuations will collapse gravitationally to form dense axion miniclusters that trap up to $\sim$75% of the dark matter within asteroid-mass clumps. Axion miniclusters are physically tiny however, so haloscope experiments searching for axions directly on Earth are much more likely to be probing ``minivoids'' -- the space in between miniclusters. This scenario seems like it ought to spell doom for haloscopes, but while these minivoids might be underdense, they are not totally devoid of axions. Using Schr\"odinger-Poisson and N-body simulations to evolve from realistic initial field configurations, we quantify the extent to which the local ambient dark matter density is suppressed in the post-inflationary scenario. We find that a typical experimental measurement will sample an axion density that is only around 10% of the expected galactic dark matter density. Our results also have implications for experimental campaigns lasting longer than a few years, as well as broadband haloscopes that have sensitivity to transient signatures. We show that for a $\mathcal{O}$(year)-long integration times, the measured dark matter density should be expected to vary by 20--30%.

Constraining axion and compact dark matter with interstellar medium heating

Cold interstellar gas systems have been used to constrain dark matter (DM) models by the condition that the heating rate from DM must be lower than the astrophysical cooling rate of the gas. Following the methodology of Wadekar and Farrar [1], we use the interstellar medium of a gas-rich dwarf galaxy, Leo T, and a Milky Way-environment gas cloud, G33.4-8.0 to constrain DM. Leo T is a particularly strong system as its gas can have the lowest cooling rate among all the objects in the late Universe (owing to the low volume density and metallicity of the gas). Milky Way clouds, in some cases, provide complementary limits as the DM-gas relative velocity in them is much larger than that in Leo T. We derive constraints on the following scenarios in which DM can heat the gas: (i) interaction of axions with hydrogen atoms or free electrons in the gas, (ii) deceleration of relic magnetically charged DM in gas plasma, (iii) dynamical friction from compact DM, (iv) hard sphere scattering of composite DM with gas. Our limits are complementary to DM direct detection searches. Detection of more gas-rich low-mass dwarfs like Leo T from upcoming 21 cm and optical surveys can improve our bounds.