Results from a search for cosmic axions

Dark Matter Axion Search with HAYSTAC Phase II

This Letter reports new results from the HAYSTAC experiment's search for dark matter axions in our galactic halo. It represents the widest search to date that utilizes squeezing to realize subquantum limited noise. The new results cover 1.71  μeV of newly scanned parameter space in the mass ranges 17.28-18.44  μeV and 18.71-19.46  μeV. No statistically significant evidence of an axion signal was observed, excluding couplings |g_{γ}|≥2.75×|g_{γ}^{KSVZ}| and |g_{γ}|≥2.96×|g_{γ}^{KSVZ}| at the 90% confidence level over the respective region. By combining this data with previously published results using HAYSTAC's squeezed state receiver, a total of 2.27  μeV of parameter space has now been scanned between 16.96-19.46  μeVμ  eV, excluding |g_{γ}|≥2.86×|g_{γ}^{KSVZ}| at the 90% confidence level. These results demonstrate the squeezed state receiver's ability to probe axion models over a significant mass range while achieving a scan rate enhancement relative to a quantum-limited experiment.

Possible Implications of QCD Axion Dark Matter Constraints from Helioscopes and Haloscopes for the String Theory Landscape

Laboratory experiments have the capacity to detect the QCD axion in the next decade, and precisely measure its mass, if it composes the majority of the dark matter. In type IIB string theory on Calabi-Yau threefolds in the geometric regime, the QCD axion mass, m_{a}, is strongly correlated with the topological Hodge number h^{1,1}. We compute m_{a} in a scan of 185965 compactifications of type IIB string theory on toric hypersurface Calabi-Yau threefolds. We compute the range of h^{1,1} probed by different experiments under the condition that the QCD axion can provide the observed dark matter density with minimal fine-tuning. Taking the experiments DMRadio, ADMX, MADMAX, and BREAD as indicative on different mass ranges, the h^{1,1} distributions peak near h^{1,1}=24.9, 65.4, 196.8, and 310.9, respectively. We furthermore conclude that, without severe fine-tuning, detection of the QCD axion as dark matter at any mass disfavors 80% of models with h^{1,1}=491, which is thought to have the most known Calabi-Yau threefolds. Measurement of the solar axion mass with IAXO is the dominant probe of all models with h^{1,1}≳250. This Letter demonstrates the immense importance of axion detection in experimentally constraining the string landscape.

Search for Dark Matter Axions with Tunable TM_{020} Mode

Axions are hypothesized particles believed to potentially resolve two major puzzles in modern physics: the strong CP problem and the nature of dark matter. Cavity-based axion haloscopes represent the most sensitive tools for probing their theoretically favored couplings to photons in the microelectronvolt range. However, as the search mass (or frequency) increases, the detection efficiency decreases, largely due to a decrease in cavity volume. Despite the potential of higher-order resonant modes to preserve experimental volume, their practical application in searches has been limited by the challenge of maintaining a high form factor over a reasonably wide search bandwidth. We introduce an innovative tuning method that uses the unique properties of auxetic materials, designed to effectively tune higher modes. This approach was applied to the TM_{020} mode for a dark matter axion search exploring a mass range from 21.38 to 21.79  μeV, resulting in the establishment of new exclusion limits for axion-photon coupling greater than approximately 10^{-13}  GeV^{-1}. These findings should allow use of higher-order modes for cavity haloscope searches.

Pulsar Nulling and Vacuum Radio Emission from Axion Clouds

Nonrelativistic axions can be efficiently produced in the polar caps of pulsars, resulting in the formation of a dense cloud of gravitationally bound axions. Here, we investigate the interplay between such an axion cloud and the electrodynamics in the pulsar magnetosphere, focusing specifically on the dynamics in the polar caps, where the impact of the axion cloud is expected to be most pronounced. For sufficiently light axions m_{a}≲10^{-7}  eV, we show that the axion cloud can occasionally screen the local electric field responsible for particle acceleration and pair production, inducing a periodic nulling of the pulsar's intrinsic radio emission. At larger axion masses, the small-scale fluctuations in the axion field tend to suppress the backreaction of the axion on the electrodynamics; however, we point out that the incoherent oscillations of the axion in short-lived regions of vacuum near the neutron star surface can produce a narrow radio line, which provides a complementary source of radio emission to the plasma-resonant emission processes identified in previous work. While this Letter focuses on the leading order correction to pair production in the magnetosphere, we speculate that there can exist dramatic deviations in the electrodynamics of these systems when the axion backreaction becomes nonlinear.

Experimental Search for Invisible Dark Matter Axions around 22 μeV

The axion has emerged as the most attractive solution to two fundamental questions in modern physics related to the charge-parity invariance in strong interactions and the invisible matter component of our Universe. Over the past decade, there have been many theoretical efforts to constrain the axion mass based on various cosmological assumptions. Interestingly, different approaches from independent groups produce good overlap between 20 and 30  μeV. We performed an experimental search to probe the presence of dark matter axions within this particular mass region. The experiment utilized a multicell cavity haloscope embedded in a 12 T magnetic field to seek for microwave signals induced by the axion-photon coupling. The results ruled out the KSVZ axions as dark matter over a mass range between 21.86 and 22.00  μeV at a 90% confidence level. This represents a sensitive experimental search guided by specific theoretical predictions.

Novel Constraints on Axions Produced in Pulsar Polar-Cap Cascades

Axions can be copiously produced in localized regions of neutron star magnetospheres where the ambient plasma is unable to efficiently screen the induced electric field. As these axions stream away from the neutron star they can resonantly transition into photons, generating a large broadband contribution to the neutron star's intrinsic radio flux. In this Letter, we develop a comprehensive end-to-end framework to model this process from the initial production of axions to the final detection of radio photons, and derive constraints on the axion-photon coupling, g_{aγγ}, using observations of 27 nearby pulsars. We study the modeling uncertainty in the sourced axion spectrum by comparing predictions from 2.5 dimensional particle-in-cell simulations with those derived using a semianalytic model; these results show remarkable agreement, leading to constraints on the axion-photon coupling that typically differ by a factor of no more than ∼2. The limits presented here are the strongest to date for axion masses 10^{-8}  eV≲m_{a}≲10^{-5}  eV, and crucially do not rely on the assumption that axions are dark matter.

Extended Axion Dark Matter Search Using the CAPP18T Haloscope

We report an extended search for the axion dark matter using the CAPP18T haloscope. The CAPP18T experiment adopts innovative technologies of a high-temperature superconducting magnet and a Josephson parametric converter. The CAPP18T detector was reconstructed after an unexpected incident of the high-temperature superconducting magnet quenching. The system reconstruction includes rebuilding the magnet, improving the impedance matching in the microwave chain, and mechanically readjusting the tuning rod to the cavity for improved thermal contact. The total system noise temperature is ∼0.6  K. The coupling between the cavity and the strong antenna is maintained at β≃2 to enhance the axion search scanning speed. The scan frequency range is from 4.8077 to 4.8181 GHz. No significant indication of the axion dark matter signature is observed. The results set the best upper bound of the axion-photon-photon coupling (g_{aγγ}) in the mass ranges of 19.883 to 19.926  μeV at ∼0.7×|g_{aγγ}^{KSVZ}| or ∼1.9×|g_{aγγ}^{DFSZ}| with 90% confidence level. The results demonstrate that a reliable search of the high-mass dark matter axions can be achieved beyond the benchmark models using the technology adopted in CAPP18T.

Direct Detection of Dark Photon Dark Matter Using Radio Telescopes

Dark photons can be the ultralight dark matter candidate, interacting with Standard Model particles via kinetic mixing. We propose to search for ultralight dark photon dark matter (DPDM) through the local absorption at different radio telescopes. The local DPDM can induce harmonic oscillations of electrons inside the antenna of radio telescopes. It leads to a monochromatic radio signal and can be recorded by telescope receivers. Using the observation data from the FAST telescope, the upper limit on the kinetic mixing can already reach 10^{-12} for DPDM oscillation frequencies at 1-1.5 GHz, which is stronger than the cosmic microwave background constraint by about one order of magnitude. Furthermore, large-scale interferometric arrays like LOFAR and SKA1 telescopes can achieve extraordinary sensitivities for direct DPDM search from 10 MHz to 10 GHz.

Near-Quantum-Noise Axion Dark Matter Search at CAPP around 9.5 μeV

We report the results of an axion dark matter search over an axion mass range of 9.39-9.51  μeV. A flux-driven Josephson parametric amplifier (JPA) was added to the cryogenic receiver chain. A system noise temperature of as low as 200 mK was achieved, which is the lowest recorded noise among published axion cavity experiments with phase-insensitive JPA operation. In addition, we developed a two-stage scanning method which boosted the scan speed by 26%. As a result, a range of two-photon coupling in a plausible model for the QCD axion was excluded with an order of magnitude higher in sensitivity than existing limits.

Search for Dark Matter Axions with CAST-CAPP

The CAST-CAPP axion haloscope, operating at CERN inside the CAST dipole magnet, has searched for axions in the 19.74 μeV to 22.47 μeV mass range. The detection concept follows the Sikivie haloscope principle, where Dark Matter axions convert into photons within a resonator immersed in a magnetic field. The CAST-CAPP resonator is an array of four individual rectangular cavities inserted in a strong dipole magnet, phase-matched to maximize the detection sensitivity. Here we report on the data acquired for 4124 h from 2019 to 2021. Each cavity is equipped with a fast frequency tuning mechanism of 10 MHz/ min between 4.774 GHz and 5.434 GHz. In the present work, we exclude axion-photon couplings for virialized galactic axions down to gaγγ = 8 × 10-14 GeV-1 at the 90% confidence level. The here implemented phase-matching technique also allows for future large-scale upgrades.

First Results from the Taiwan Axion Search Experiment with a Haloscope at 19.6 μeV

This Letter reports on the first results from the Taiwan Axion Search Experiment with a Haloscope, a search for axions using a microwave cavity at frequencies between 4.707 50 and 4.798 15 GHz. Apart from the nonaxion signals, no candidates with a significance of more than 3.355 were found. The experiment excludes models with the axion-two-photon coupling |g_{aγγ}|≳8.1×10^{-14}  GeV^{-1}, a factor of eleven above the benchmark Kim-Shifman-Vainshtein-Zakharov model, in the mass range 19.4687<m_{a}<19.8436  μeV. It is also the first time that a haloscope experiment places constraints on g_{aγγ} in the mass region of 19.4687<m_{a}<19.7639  μeV, reaching a sensitivity 3 orders of magnitude better than the limits obtained by nonhaloscope experiments.

Searching for Invisible Axion Dark Matter with an 18 T Magnet Haloscope

We report the first search results for axion dark matter using an 18 T high-temperature superconducting magnet haloscope. The scan frequency ranges from 4.7789 to 4.8094 GHz. No significant signal consistent with the Galactic halo dark matter axion is observed. The results set the best upper bound of axion-photon-photon coupling (g_{aγγ}) in the mass ranges of 19.764 to 19.771  μeV (19.863 to 19.890  μeV) at 1.5×|g_{aγγ}^{KSVZ}| (1.7×|g_{aγγ}^{KSVZ}|), and 19.772 to 19.863  μeV at 2.7×|g_{aγγ}^{KSVZ}| with 90% confidence level, respectively. This remarkable sensitivity in the high mass region of dark matter axion is achieved by using the strongest magnetic field among the existing haloscope experiments and realizing a low-noise amplification of microwave signals using a Josephson parametric converter.

Direct detection of dark matter-APPEC committee report

This report provides an extensive review of the experimental programme of direct detection searches of particle dark matter. It focuses mostly on European efforts, both current and planned, but does it within a broader context of a worldwide activity in the field. It aims at identifying the virtues, opportunities and challenges associated with the different experimental approaches and search techniques. It presents scientific and technological synergies, both existing and emerging, with some other areas of particle physics, notably collider and neutrino programmes, and beyond. It addresses the issue of infrastructure in light of the growing needs and challenges of the different experimental searches. Finally, the report makes a number of recommendations from the perspective of a long-term future of the field. They are introduced, along with some justification, in the opening overview and recommendations section and are next summarised at the end of the report. Overall, we recommend that the direct search for dark matter particle interactions with a detector target should be given top priority in astroparticle physics, and in all particle physics, and beyond, as a positive measurement will provide the most unambiguous confirmation of the particle nature of dark matter in the Universe.

Broadband Solenoidal Haloscope for Terahertz Axion Detection

We introduce the Broadband Reflector Experiment for Axion Detection (BREAD) conceptual design and science program. This haloscope plans to search for bosonic dark matter across the [10^{-3},1]  eV ([0.24, 240] THz) mass range. BREAD proposes a cylindrical metal barrel to convert dark matter into photons, which a novel parabolic reflector design focuses onto a photosensor. This unique geometry enables enclosure in standard cryostats and high-field solenoids, overcoming limitations of current dish antennas. A pilot 0.7  m^{2} barrel experiment planned at Fermilab is projected to surpass existing dark photon coupling constraints by over a decade with one-day runtime. Axion sensitivity requires <10^{-20}  W/sqrt[Hz] sensor noise equivalent power with a 10 T solenoid and 10  m^{2} barrel. We project BREAD sensitivity for various sensor technologies and discuss future prospects.

Axion dark matter: How to see it?

The axion is a highly motivated elementary particle that could address two fundamental questions in physics-the strong charge-parity (CP) problem and the dark matter mystery. Experimental searches for this hypothetical particle started reaching theoretically interesting sensitivity levels, particularly in the micro-electron volt (gigahertz) region. They rely on microwave resonators in strong magnetic fields with signals read out by quantum noise limited amplifiers. Concurrently, there have been intensive experimental efforts to widen the search range by devising various techniques and to enhance sensitivities by implementing advanced technologies. These orthogonal approaches will enable us to explore most of the parameter space for axions and axion-like particles within the next decades, with the 1- to 25-gigahertz frequency range to be conquered well within the first decade. We review the experimental aspects of axion physics and discuss the past, present, and future of the direct search programs.

Design of New Resonant Haloscopes in the Search for the Dark Matter Axion: A Review of the First Steps in the RADES Collaboration

With the increasing interest in dark matter axion detection through haloscopes, in which different international groups are currently involved, the RADES group was established in 2016 with the goal of developing very sensitive detection systems to be operated in dipole magnets. This review deals with the work developed by this collaboration during its first five years: from the first designs—based on the multi-cavity concept, aiming to increase the haloscope volume, and thereby improve sensitivity—to their evolution, data acquisition design, and finally, the first experimental run. Moreover, the envisaged work within RADES for both dipole and solenoid magnets in the short and medium term is also presented.

Stochastic fluctuations of bosonic dark matter

Numerous theories extending beyond the standard model of particle physics predict the existence of bosons that could constitute dark matter. In the standard halo model of galactic dark matter, the velocity distribution of the bosonic dark matter field defines a characteristic coherence time τc. Until recently, laboratory experiments searching for bosonic dark matter fields have been in the regime where the measurement time T significantly exceeds τc, so null results have been interpreted by assuming a bosonic field amplitude Φ0 fixed by the average local dark matter density. Here we show that experiments operating in the T ≪ τc regime do not sample the full distribution of bosonic dark matter field amplitudes and therefore it is incorrect to assume a fixed value of Φ0 when inferring constraints. Instead, in order to interpret laboratory measurements (even in the event of a discovery), it is necessary to account for the stochastic nature of such a virialized ultralight field. The constraints inferred from several previous null experiments searching for ultralight bosonic dark matter were overestimated by factors ranging from 3 to 10 depending on experimental details, model assumptions, and choice of inference framework.

The Echo Method for Axion Dark Matter Detection

The axion is a dark matter candidate arising from the spontaneous breaking of the Peccei–Quinn symmetry, introduced to solve the strong CP problem. It has been shown that radio/microwave radiation sent out to space is backscattered in the presence of axion dark matter due to stimulated axion decay. This backscattering is a feeble and narrow echo signal centered at an angular frequency very close to one-half of the axion mass. In this article, we summarize all the relevant results found so far, including analytical formulas for the echo signal, as well as sensitivity prospects for possible near-future experiments.

Search for Low-Mass Axion Dark Matter with ABRACADABRA-10 cm

Two of the most pressing questions in physics are the microscopic nature of the dark matter that comprises 84% of the mass in the Universe and the absence of a neutron electric dipole moment. These questions would be resolved by the existence of a hypothetical particle known as the quantum chromodynamics (QCD) axion. In this work, we probe the hypothesis that axions constitute dark matter, using the ABRACADABRA-10 cm experiment in a broadband configuration, with world-leading sensitivity. We find no significant evidence for axions, and we present 95% upper limits on the axion-photon coupling down to the world-leading level g_{aγγ}<3.2×10^{-11}  GeV^{-1}, representing one of the most sensitive searches for axions in the 0.41-8.27 neV mass range. Our work paves a direct path for future experiments capable of confirming or excluding the hypothesis that dark matter is a QCD axion in the mass range motivated by string theory and grand unified theories.

Erratum: Upconversion Loop Oscillator Axion Detection Experiment: A Precision Frequency Interferometric Axion Dark Matter Search with a Cylindrical Microwave Cavity [Phys. Rev. Lett. 126, 081803 (2021)]

This corrects the article DOI: 10.1103/PhysRevLett.126.081803.

First Results from an Axion Haloscope at CAPP around 10.7 μeV

The Center for Axion and Precision Physics Research at the Institute for Basic Science is searching for axion dark matter using ultralow temperature microwave resonators. We report the exclusion of the axion mass range 10.7126-10.7186  μeV with near Kim-Shifman-Vainshtein-Zakharov (KSVZ) coupling sensitivity and the range 10.16-11.37  μeV with about 9 times larger coupling at 90% confidence level. This is the first axion search result in these ranges. It is also the first with a resonator physical temperature of less than 40 mK.

Radio-frequency Dark Photon Dark Matter across the Sun

The Dark photon as an ultralight dark matter candidate can interact with the standard model particles via kinetic mixing. We propose to search for the ultralight dark photon dark matter using radio telescopes with solar observations. The dark photon dark matter can efficiently convert into photons in the outermost region of the solar atmosphere, the solar corona, where the plasma mass of photons is close to the dark photon rest mass. Because of the strong resonant conversion and benefiting from the short distance between the Sun and the Earth, the radio telescopes can lead the dark photon search sensitivity in the mass range of 4×10^{-8}-4×10^{-6}  eV, corresponding to the frequency 10-1000 MHz. As a promising example, the low-frequency array telescope can reach the kinetic mixing ε∼10^{-13} (10^{-14}) within 1 (100) h of solar observations. The future experiment square kilometer array phase 1 can reach ε∼10^{-16}-10^{-14} with 1 h of solar observations.

Upconversion Loop Oscillator Axion Detection Experiment: A Precision Frequency Interferometric Axion Dark Matter Search with a Cylindrical Microwave Cavity

First experimental results from a room-temperature tabletop phase-sensitive axion haloscope experiment are presented. The technique exploits the axion-photon coupling between two photonic resonator oscillators excited in a single cavity, allowing low-mass axions to be upconverted to microwave frequencies, acting as a source of frequency modulation on the microwave carriers. This new pathway to axion detection has certain advantages over the traditional haloscope method, particularly in targeting axions below 1  μeV (240 MHz) in energy. At the heart of the dual-mode oscillator, a tunable cylindrical microwave cavity supports a pair of orthogonally polarized modes (TM_{0,2,0} and TE_{0,1,1}), which, in general, enables simultaneous sensitivity to axions with masses corresponding to the sum and difference of the microwave frequencies. However, in the reported experiment, the configuration was such that the sum frequency sensitivity was suppressed, while the difference frequency sensitivity was enhanced. The results place axion exclusion limits between 7.44-19.38 neV, excluding a minimal coupling strength above 5×10^{-7}  1/GeV, after a measurement period of two and a half hours. We show that a state-of-the-art frequency-stabilized cryogenic implementation of this technique, ambitious but realizable, may achieve the best limits in a vast range of axion space.

Constraints on the Coupling between Axionlike Dark Matter and Photons Using an Antiproton Superconducting Tuned Detection Circuit in a Cryogenic Penning Trap

We constrain the coupling between axionlike particles (ALPs) and photons, measured with the superconducting resonant detection circuit of a cryogenic Penning trap. By searching the noise spectrum of our fixed-frequency resonant circuit for peaks caused by dark matter ALPs converting into photons in the strong magnetic field of the Penning-trap magnet, we are able to constrain the coupling of ALPs with masses around 2.7906-2.7914  neV/c^{2} to g_{aγ}<1×10^{-11}  GeV^{-1}. This is more than one order of magnitude lower than the best laboratory haloscope and approximately 5 times lower than the CERN axion solar telescope (CAST), setting limits in a mass and coupling range which is not constrained by astrophysical observations. Our approach can be extended to many other Penning-trap experiments and has the potential to provide broad limits in the low ALP mass range.

Search for Invisible Axion Dark Matter with a Multiple-Cell Haloscope

We present the first results of a search for invisible axion dark matter using a multiple-cell cavity haloscope. This cavity concept was proposed to provide a highly efficient approach to high-mass regions compared to the conventional multiple-cavity design, with larger detection volume, simpler detector setup, and a unique phase-matching mechanism. Searches with a double-cell cavity superseded previous reports for the axion-photon coupling over the mass range between 13.0 and 13.9  μeV. This result not only demonstrates the novelty of the cavity concept for high-mass axion searches, but also suggests it can make considerable contributions to the next-generation experiments.

Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter Conversion in Neutron Star Magnetospheres

Axion dark matter (DM) may convert to radio-frequency electromagnetic radiation in the strong magnetic fields around neutron stars. The radio signature of such a process would be an ultranarrow spectral peak at a frequency determined by the mass of the axion particle. We analyze data we collected from the Robert C. Byrd Green Bank Telescope in the L band and the Effelsberg 100-m Telescope in the L band and S band from a number of sources expected to produce bright signals of axion-photon conversion, including the Galactic center of the Milky Way and the nearby isolated neutron stars RX J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able to set constraints on the existence of axion DM in the highly motivated mass range between ∼5 and 11  μeV with the strongest constraints to date on axions in the ∼10-11  μeV range.

High quality factor photonic cavity for dark matter axion searches

Searches for dark matter axions involve the use of microwave resonant cavities operating in a strong magnetic field. Detector sensitivity is directly related to the cavity quality factor, which is limited, until recently, to the use of non-superconducting metals by the presence of the external magnetic field. In this paper, we present a cavity of novel design whose quality factor is not affected by a magnetic field. It is based on a photonic structure by the use of sapphire rods. The quality factor at cryogenic temperature is in excess of 5 × 10⁵ for a selected mode.

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 (50 MHz) 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 (42.31-42.36 MHz), 1.7734-1.7738×10^{-7}  eV (42.88-42.89 MHz), and 1.8007-1.8015×10^{-7}  eV (43.54-43.56 MHz) were covered at magnetic fields of 4.5 T, 5.0 T, and 7.0 T, respectively. Exclusion results from the search data, for coupling below 10^{-12}  GeV^{-1}, are presented.

Axion Dark Matter Search around 6.7 μeV

An axion dark matter search with the CAPP-8TB haloscope is reported. Our results are sensitive to axion-photon coupling g_{aγγ} down to the QCD axion band over the axion mass range between 6.62 and 6.82  μeV at a 90% confidence level, which is the most sensitive result in the mass range to date.

Tunable Axion Plasma Haloscopes

We propose a new strategy for searching for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes, we estimate competitive sensitivity for axion masses of 35-400  μeV, at least.

Direct Searches for Hidden-Photon Dark Matter with the SHUKET Experiment

Hidden photons are dark matter candidates motivated by theories beyond the standard model of particle physics. They mix with conventional photons, and they can be detected through the very weak electromagnetic radiation they induce at the interface between a metal and the air. SHUKET [search for U(1) dark matter with an electromagnetic telescope] is a dedicated experiment sensitive to the hidden photon-induced signal. The results from a hidden photon search campaign are reported, with no significant detection of a dark matter signal. Exclusion limits are derived from the observed noise fluctuations in a 5-6.8 GHz frequency range, corresponding to a hidden photon mass region ranging from 20.8 to 28.3  μeV. SHUKET is currently the most sensitive instrument in this mass range, and the obtained limits on the kinetic mixing term constrain significantly dark matter models inspired from string theory.

First Results from ABRACADABRA-10 cm: A Search for Sub-μeV Axion Dark Matter

The axion is a promising dark matter candidate, which was originally proposed to solve the strong-CP problem in particle physics. To date, the available parameter space for axion and axionlike particle dark matter is relatively unexplored, particularly at masses m_{a}≲1  μeV. ABRACADABRA is a new experimental program to search for axion dark matter over a broad range of masses, 10^{-12}≲m_{a}≲10^{-6}  eV. ABRACADABRA-10 cm is a small-scale prototype for a future detector that could be sensitive to the QCD axion. In this Letter, we present the first results from a 1 month search for axions with ABRACADABRA-10 cm. We find no evidence for axionlike cosmic dark matter and set 95% C.L. upper limits on the axion-photon coupling between g_{aγγ}<1.4×10^{-10} and g_{aγγ}<3.3×10^{-9}  GeV^{-1} over the mass range 3.1×10^{-10}-8.3×10^{-9}  eV. These results are competitive with the most stringent astrophysical constraints in this mass range.

Piezoelectrically Tuned Multimode Cavity Search for Axion Dark Matter

The μeV axion is a well-motivated extension to the standard model. The Axion Dark Matter eXperiment (ADMX) collaboration seeks to discover this particle by looking for the resonant conversion of dark-matter axions to microwave photons in a strong magnetic field. In this Letter, we report results from a pathfinder experiment, the ADMX "Sidecar," which is designed to pave the way for future, higher mass, searches. This testbed experiment lives inside of and operates in tandem with the main ADMX experiment. The Sidecar experiment excludes masses in three widely spaced frequency ranges (4202-4249, 5086-5799, and 7173-7203 MHz). In addition, Sidecar demonstrates the successful use of a piezoelectric actuator for cavity tuning. Finally, this publication is the first to report data measured using both the TM_{010} and TM_{020} modes.

Search for Invisible Axion Dark Matter with the Axion Dark Matter Experiment

This Letter reports the results from a haloscope search for dark matter axions with masses between 2.66 and 2.81  μeV. The search excludes the range of axion-photon couplings predicted by plausible models of the invisible axion. This unprecedented sensitivity is achieved by operating a large-volume haloscope at subkelvin temperatures, thereby reducing thermal noise as well as the excess noise from the ultralow-noise superconducting quantum interference device amplifier used for the signal power readout. Ongoing searches will provide nearly definitive tests of the invisible axion model over a wide range of axion masses.

Dielectric Haloscopes: A New Way to Detect Axion Dark Matter

We propose a new strategy to search for dark matter axions in the mass range of 40-400 μeV by introducing dielectric haloscopes, which consist of dielectric disks placed in a magnetic field. The changing dielectric media cause discontinuities in the axion-induced electric field, leading to the generation of propagating electromagnetic waves to satisfy the continuity requirements at the interfaces. Large-area disks with adjustable distances boost the microwave signal (10-100 GHz) to an observable level and allow one to scan over a broad axion mass range. A sensitivity to QCD axion models is conceivable with 80 disks of 1  m^{2} area contained in a 10 T field.

First Results from a Microwave Cavity Axion Search at 24 μeV

We report on the first results from a new microwave cavity search for dark matter axions with masses above 20  μeV. We exclude axion models with two-photon coupling g_{aγγ}≳2×10^{-14}  GeV^{-1} over the range 23.55<m_{a}<24.0  μeV. These results represent two important achievements. First, we have reached cosmologically relevant sensitivity an order of magnitude higher in mass than any existing limits. Second, by incorporating a dilution refrigerator and Josephson parametric amplifier, we have demonstrated total noise approaching the standard quantum limit for the first time in an axion search.

Photon regeneration experiment for axion search using x-rays

In this Letter we describe our novel photon regeneration experiment for the axionlike particle search using an x-ray beam with a photon energy of 50.2 and 90.7 keV, two superconducting magnets of 3 T, and a Ge detector with a high quantum efficiency. A counting rate of regenerated photons compatible with zero has been measured. The corresponding limits on the pseudoscalar axionlike particle-two-photon coupling constant is obtained as a function of the particle mass. Our setup widens the energy window of purely terrestrial experiments devoted to the axionlike particle search by coupling to two photons. It also opens a new domain of experimental investigation of photon propagation in magnetic fields.

Proposal for a search for cosmic axions using an optical cavity

A high finesse optical cavity can be used to search for cosmic axions in the mass range 10;{-6} < m_{a} < 10;{-4} eV. Either a two-arm or a single-arm cavity is suitable, and in either case the signal appears as resonant sidebands imposed on the carrier. Assuming for the local axion density the usual figure of rho_{a} = 500 Mev/cm;{3}, the KSVZ axion line g_{agammagamma}/m_{a} = 0.4 GeV-2, can be reached over the full mass range in a 1 yr search.

Experimental searches for galactic halo axions

A very light axion would be copiously produced during the Big Bang as a zero-temperature Bose gas, and it would possess vanishingly small couplings to matter and radiation. It thus represents an ideal cold dark matter candidate. Galactic halo axions may be detected by their resonant conversion to microwave photons in a high-Q cavity permeated by a strong magnetic field. A large-scale search for the axion is ongoing in the US with sufficient sensitivity to see axions of plausible model couplings. Dramatic breakthroughs in the development of near-quantum limited superconducting quantum interference device amplifiers promise to improve the sensitivity of the experiment by a factor of 30 in the near future. In Japan, a group has been developing a Rydberg atom single-quantum detector as an alternative to linear amplifiers for a microwave-cavity axion experiment. Should the axion be discovered, the predicted fine structure in the axion signal would be rich in information about the history of galactic formation.

Direct searches for dark matter: recent results

There is abundant evidence for large amounts of unseen matter in the universe. This dark matter, by its very nature, couples feebly to ordinary matter and is correspondingly difficult to detect. Nonetheless, several experiments are now underway with the sensitivity required to detect directly galactic halo dark matter through their interactions with matter and radiation. These experiments divide into two broad classes: searches for weakly interacting massive particles (WIMPs) and searches for axions. There exists a very strong theoretical bias for supposing that supersymmetry (SUSY) is a correct description of nature. WIMPs are predicted by this SUSY theory and have the required properties to be dark matter. These WIMPs are detected from the byproducts of their occasional recoil against nucleons. There are efforts around the world to detect these rare recoils. The WIMP part of this overview focuses on the cryogenic dark matter search (CDMS) underway in California. Axions, another favored dark matter candidate, are predicted to arise from a minimal extension of the standard model that explains the absence of the expected large CP violating effects in strong interactions. Axions can, in the presence of a large magnetic field, turn into microwave photons. It is the slight excess of photons above noise that signals the axion. Axion searches are underway in California and Japan. The axion part of this overview focuses on the California effort. Brevity does not allow me to discuss other WIMP and axion searches, likewise for accelerator and satellite based searches; I apologize for their omission.