Axion Dark Matter Research Articles

Resonant instability of axionic dark matter clumps

Axion is a popular candidate for dark matter particles. Axionic dark matter may form Bose-Einstein condensate and may be gravitationally bound to form axion clumps. Under the presence of electromagnetic waves with frequency ω= ma/ 2, where ma is the axion mass, a resonant enhancement may occur, causing an instability of the axion clumps. With analytical and numerical approaches, we study the resonant instability of axionic dark matter clumps with infinite homogeneous mass distribution, as well as distribution with a finite boundary. After taking realistic astrophysical environments into consideration, including gravitational redshift and plasma effects, we obtain an instability region in the axion density—clump size parameter space with given mass and coupling of axions. In particular, we show that, for axion clumps formed by the QCD axions in equilibrium, no resonant instability will occur.

Dark matter axion detection in the radio/mm waveband

We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic centre or the Virgo cluster centre offers the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetised plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a non-perturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and non-relativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the non-adiabatic limit. Our treatment reveals that that infalling dark matter axions typically convert non-adiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarisations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal is dominated by Doppler broadening from the relative motion of the neutron star with respect to the observer. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings.

Radio-emission of axion stars

We study parametric instability of compact axion dark matter structures decaying to radiophotons. Corresponding objects - Bose (axion) stars, their clusters, and clouds of diffuse axions - form abundantly in the postinflationary Peccei-Quinn scenario. We develop general description of parametric resonance incorporating finite-volume effects, backreaction, axion velocities and their (in)coherence. With additional coarse-graining, our formalism reproduces kinetic equation for virialized axions interacting with photons. We derive conditions for the parametric instability in each of the above objects, as well as in collapsing axion stars, evaluate photon resonance modes and their growth exponents. As a by-product, we calculate stimulated emission of Bose stars and diffuse axions, arguing that the former can give larger contribution into the radiobackground. In the case of QCD axions, the Bose stars glow and collapsing stars radioburst if the axion-photon coupling exceeds the original KSVZ value by two orders of magnitude. The latter constraint is alleviated for several nearby axion stars in resonance and absent for axion-like particles. Our results show that the parametric effect may reveal itself in observations, from FRB to excess radiobackground.

Inflaton clusters and inflaton stars

In a broad class of scenarios, inflation is followed by an extended era of matter-dominated expansion during which the inflaton condensate is nonrelativistic on subhorizon scales. During this phase density perturbations grow to the point of nonlinearity and collapse into bound structures. This epoch strongly resembles structure formation with ultra-light axion-like particles. This parallel permits us to adapt results from studies of cosmological structure formation to describe the nonlinear dynamics of this post-inflationary epoch. We show that the inflaton condensate fragments into “inflaton clusters”, analogues of axion dark matter halos in present-day cosmology. Moreover, solitonic objects or “inflaton stars” can form inside these clusters, leading to density contrasts as large as 106 in the post-inflationary universe.

Scalable haloscopes for axion dark matter detection in the 30 \u03bceV range with RADES

RADES (Relic Axion Detector Exploratory Setup) is a project with the goal of directly searching for axion dark matter above the 30μeV scale employing custom-made microwave filters in magnetic dipole fields. Currently RADES is taking data at the LHC dipole of the CAST experiment. In the long term, the RADES cavities are envisioned to take data in the BabyIAXO magnet. In this article we report on the modelling, building and characterisation of an optimised microwave-filter design with alternating irises that exploits maximal coupling to axions while being scalable in length without suffering from mode-mixing. We develop the mathematical formalism and theoretical study which justifies the performance of the chosen design. We also point towards the applicability of this formalism to optimise the MADMAX dielectric haloscopes.

Merger of dark matter axion clumps and resonant photon emission

A portion of light scalar dark matter, especially axions, may organize into gravitationally bound clumps (stars) and be present in large number in the galaxy today. It is therefore of utmost interest to determine if there are novel observational signatures of this scenario. Work has shown that for moderately large axion-photon couplings, such clumps can undergo parametric resonance into photons, for clumps above a critical mass M⋆c determined precisely by some of us in ref. [1]. In order to obtain a clump above the critical mass in the galaxy today would require mergers. In this work we perform full 3-dimensional simulations of pairs of axion clumps and determine the conditions under which mergers take place through the emission of scalar waves, including analyzing head-on and non-head-on collisions, phase dependence, and relative velocities. Consistent with other work in the literature, we find that the final mass from the merger M⋆final≈ 0.7(M⋆1+M⋆2) is larger than each of the original clump masses (for M⋆1∼ M⋆2). Hence, it is possible for sub-critical mass clumps to merge and become super-critical and therefore undergo parametric resonance into photons. We find that mergers are expected to be kinematically allowed in the galaxy today for high Peccei-Quinn scales, which is strongly suggested by unification ideas, although the collision rate is small. While mergers can happen for axions with lower Peccei-Quinn scales due to statistical fluctuations in relative velocities, as they have a high collision rate. We estimate the collision and merger rates within the Milky Way galaxy today. We find that a merger leads to a flux of energy on earth that can be appreciable and we mention observational search strategies.

Gravitational Waves in Axion Dark Matter

Axion dark matter is interesting as it allows a natural coupling to the gravitational Chern–Simons term. In the presence of an axion background, the gravitational Chern–Simons term produces parity violating effects in the gravitational sector, in particular on the propagation of gravitational waves. Previously, it has been shown that the coherent oscillation of the axion field leads to a parametric amplification of gravitational waves with a specific frequency. In this paper, we focus on the parity violating effects of the Chern–Simons coupling and show the occurrence of gravitational birefringence. We also find deviation from the speed of light of the velocity of the gravitational waves. We give constraints on the axion-Chern–Simons coupling constant and the abundance of axion dark matter from the observation of GW170817 and GRB170817A.

Axion Kinetic Misalignment Mechanism.

In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter. This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate.

Gravitational interactions and neutrino masses

We describe a scenario where the smallness of neutrino masses is related to a global symmetry that is only violated by quantum gravitational effects. The coupling of neutrinos to gauge singlet right-handed fermions is attributed to symmetry preserving gravitational operators suppressed by the Planck mass, in this framework. The proposed scenario leads to axion particles that decay into neutrinos, which could be probed through cosmological measurements and may help explain the Hubble parameter tension. Depending on the details of the implementation, the scenario could provide axion dark matter candidates.

Multiple ultralight axionic wave dark matter and astronomical structures

An ultralight scalar boson with mass $m_1 \simeq 10^{-22}$ eV is gaining credence as a Dark Matter (DM) candidate that explains the dark cores of dwarf galaxies as soliton waves. Such a boson is naturally interpreted as an axion generic in String Theory, with multiple light axions predicted in this context. We examine the possibility of soliton structures over a wide range of scales, accounting for galaxy core masses and the common presence of nuclear star clusters. We present a diagnostic soliton core mass-radius plot that provides a global view, indicating the existence of an additional axion with mass $m_2\simeq 10^{-20}$ eV, with the possibility of a third axion with mass $m_3 \gtrsim 0.5 \times 10^{-18}$ eV. We also argue that the relative mass densities measured for these axions are consistent with their cosmological production via the mis-alignment mechanism.

ADMX SLIC: Results from a Superconducting LC Circuit Investigating Cold Axions.

Axions are a promising cold dark matter candidate. Haloscopes, which use the conversion of axions to photons in the presence of a magnetic field to detect axions, are the basis of microwave cavity searches such as the Axion Dark Matter eXperiment (ADMX). To search for lighter, low frequency axions in the sub- 2×10^{-7} eV (50MHz) range, a tunable lumped-element LC circuit has been proposed. For the first time, through ADMX SLIC (Superconducting LC Circuit Investigating Cold Axions), a resonant LC circuit was used to probe this region of axion mass-coupling space. The detector used a superconducting LC circuit with piezoelectric driven capacitive tuning. The axion mass and corresponding frequency ranges 1.7498-1.7519×10^{-7} eV (42.31-42.36MHz), 1.7734-1.7738×10^{-7} eV (42.88-42.89MHz), and 1.8007-1.8015×10^{-7} eV (43.54-43.56MHz) were covered at magnetic fields of 4.5T, 5.0T, and 7.0T, respectively. Exclusion results from the search data, for coupling below 10^{-12} GeV^{-1}, are presented.

Broadband electrical action sensing techniques with conducting wires for low-mass dark matter axion detection

Due to the inverse Primakoff effect, it has been shown that when axions mix with a DC B→-field, the resulting electrical action will produce an AC electromotive force, which oscillates at the Compton frequency of the axion. As in standard electrodynamics, this electromotive force may be modelled as an oscillating effective impressed magnetic current boundary source. We use this result to calculate the sensitivity of new experiments to low-mass axions using the quasi-static technique, defined as when the Compton wavelength of the axion is greater than the dimensions of the experiment. First, we calculate the current induced in a straight conducting wire (electric dipole antenna) in the limit where the DC B→-field can be considered as spatially constant and show that it has a sensitivity proportional to the axion mass. Following this we extend the topology by making use of the full extent of the spatially varying DC B→-field of the electromagnet. This is achieved by transforming the 1D conducting wire to a 2D winding with inductance, to fully link the effective magnetic current boundary source and hence couple to the full axion induced electrical action (or electromotive force). We investigate two different topologies: The first uses a single winding, and couples to the effective short circuit current generated in the winding, which is optimally read out using a sensitive low impedance SQUID amplifier: The second technique uses multiple windings, with every turn effectively increasing the induced voltage, which is proportional to the winding number. The read out of this configuration is optimised by implementing a cryogenic low-noise high input impedance voltage amplifier. The end result is the realisation of new Broadband Electrical Action Sensing Techniques with orders of magnitude improved sensitivity over current low-mass axion experiments, with a sensitivity linearly proportional to the axion–photon coupling and capable of detecting QCD dark matter axions in the mass range of 10−12−10−8eV and below.

Improved analysis framework for axion dark matter searches

In experiments searching for axionic dark matter, the use of the standard threshold-based data analysis discards valuable information. We present a Bayesian analysis framework that builds on an existing processing protocol to extract more information from the data of coherent axion detectors such as operating haloscopes. The analysis avoids logical subtleties that accompany the standard analysis framework and enables greater experimental flexibility on future data runs. Performing this analysis on the existing data from the HAYSTAC experiment, we find improved constraints on the axion-photon coupling $g_\gamma$ while also identifying the most promising regions of parameter space within the $23.15$--$24.0$ $\mu$eV mass range. A comparison with the standard threshold analysis suggests a $36\%$ improvement in scan rate from our analysis, demonstrating the utility of this framework for future axion haloscope analyses.

Radio signal of axion-photon conversion in neutron stars: A ray tracing analysis

Axion dark matter can resonantly convert into photons in the magnetospheres of neutron stars (NSs). It has recently been shown that radio observations of nearby NSs can therefore provide a highly sensitive probe of the axion parameter space. Here we extend existing calculations by performing the first three-dimensional computation of the photon flux, taking into account the isotropic phase-space distribution of axions and the structure of the NS magnetosphere. In particular, we study the overall magnitude of the flux and its possible time variation. We find that overall signal strength is robust to our more realistic analysis. In addition, we find that the variance of the signal with respect to the NS rotation is washed out by the additional trajectories in our treatment. Nevertheless, we show that SKA observations towards J0806.4-4123 are sensitive to $g_{a\gamma\gamma}\sim 3\times10^{-13}\mathrm{\,GeV^{-1}}$ at $m_a\sim 7\times10^{-6}\mathrm{\,eV}$, even when accounting for Doppler broadening. Finally, we provide the necessary code to calculate the photon flux for any given NS system.

QCD axion dark matter from a late time phase transition

We investigate the possibility that the Peccei-Quinn phase transition occurs at a temperature far below the symmetry breaking scale. Low phase transition temperatures are typical in supersymmetric theories, where symmetry breaking fields have small masses. We find that QCD axions are abundantly produced just after the phase transition. The observed dark matter abundance is reproduced even if the decay constant is much lower than 1011 GeV. The produced axions tend to be warm. For some range of the decay constant, the effect of the predicted warmness on structure formation can be confirmed by future observations of 21 cm lines. A portion of parameter space requires a mixing between the Peccei-Quinn symmetry breaking field and the Standard Model Higgs, and predicts an observable rate of rare Kaon decays.

A formalism for magnon gravitational wave detectors

In order to detect high frequency gravitational waves, we need a new detection method. In this paper, we develop a formalism for a gravitational wave detector using magnons in a cavity. Using Fermi normal coordinates and taking the non-relativistic limit, we obtain a Hamiltonian for magnons in gravitational wave backgrounds. Given the Hamiltonian, we show how to use the magnons for detecting high frequency gravitational waves. Furthermore, as a demonstration of the magnon gravitational wave detector, we give upper limits on GHz gravitational waves by utilizing known results of magnon experiments for an axion dark matter search.

Large-volume centimeter-wave cavities for axion searches

The scan rate of an axion haloscope is proportional to the square of the cavity volume. In this paper, a new class of thin-shell cavities are proposed to search for axionic dark matter. These cavities feature active volume much larger (>20×) than that of a conventional cylindrical haloscope, comparable quality factor Q, and a similar frequency tuning range. Full 3D numerical finite-element analyses have been used to show that the TM010 eigenmodes are singly polarized throughout the volume of the cavity and can facilitate axion-photon conversion in uniform magnetic field produced by a superconducting solenoid. To mitigate spurious mode crowding and volume losses due to localization, a pre-amplification binary summing network will be used for coupling. Because of the favorable frequency-scaling, the new cavities are most suitable for centimeter-wavelength (∼ 10–100 GHz), corresponding to the promising post-inflation axion production window. In this frequency range, the tight machining tolerances required for high-Q thin-shell cavities are achievable with standard machining techniques for near-infrared mirrors.

Scalars Gliding through an Expanding Universe.

In this Letter, we investigate the effects of single derivative mixing in massive bosonic fields. In the regime of large mixing, we show that this leads to striking changes of the field dynamics, delaying the onset of classical oscillations and decreasing, or even eliminating, the friction due to Hubble expansion. We highlight this phenomenon with a few examples. In the first example, we show how an axionlike particle can have its number abundance parametrically enhanced. In the second example, we demonstrate that the QCD axion can have its number abundance enhanced allowing for misalignment driven axion dark matter all the way down to f_{a} of order astrophysical bounds. In the third example, we show that the delayed oscillation of the scalar field can also sustain a period of inflation. In the last example, we present a situation where an oscillating scalar field is completely frictionless and does not dilute away in time.

Dynamical evolution of axion condensates under stimulated decays into photons

Dark matter axion condensates may experience stimulated decays into photon pairs. This effect has been often interpreted as a parametric resonance of photons from the axion-photon coupling, leading to an exponential growth of the photon occupation number in a narrow instability band. Most of the previous literature does not consider the possible evolution of the axion field due to the photon growth. We revisit this effect presenting a mean field solution of the axion-photon kinetic equations, in terms of number of photons and pair correlations. We study the limit of no axion depletion, recovering the known instability. Moreover, we extend the results including a possible depletion of the axion field. In this case we find that the axion condensate exhibits the behaviour of an inverted pendulum. We discuss the relevance of these effects for two different cases: an homogeneous axion field at recombination and a localized axion clump and discuss constraints that could result from the induced photon background.

Stochastic axion dark matter in axion landscape

We study the stochastic axion dark matter scenario in the axion landscape, where one of the axions is light and stable and therefore explains dark matter. If the axion mass at the potential is a typical value of the curvature along the direction, the potential can be well approximated by a quadratic mass term. On the other hand, if the axion mass happens to be suppressed in the vicinity of the minimum, the potential may be approximated by a quartic potential plus a suppressed quadratic one, for which the initial angle, and thus the axion abundance, can be significantly suppressed compared to the quadratic case. We delineate the viable parameter region by taking account of various observational constraints, and find that a broader range of the inflation scale is allowed. Also, if the curvature of the potential is suppressed over a certain range of the potential, the onset of coherent oscillations can be delayed. Then, the axion dark matter with a small decay constant is possible. We also discuss the π\\hspace{-0.2mm}nflation mechanism to realize the hilltop initial condition in the stochastic axion scenario.