Axions in the µeV mass range are a plausible cold dark matter candidate and may be detected by their conversion into microwave photons in a resonant cavity immersed in a static magnetic field. The first result from such an axion search using a superconducting first-stage amplifier (SQUID) is reported. The SQUID amplifier, replacing a conventional GaAs field-effect transistor amplifier, successfully reached axion-photon coupling sensitivity in the band set by present axion models and sets the stage for a definitive axion search utilizing near quantum-limited SQUID amplifiers.

Recent determinations of cosmological parameters point to a flat Universe, whose total energy density is composed of about two-thirds vacuum energy and one-third matter. Ordinary baryonic matter is relegated to a small fraction of the latter, within which the luminous part is an order of magnitude smaller yet. Particle dark matter, i.e., one or more relic particle species from the big bang, is thus strongly suggested as the dominant component of matter in the Universe. The axion, a hypothetical elementary pseudoscalar arising from the Peccei-Quinn solution to the strong-CP problem, is a well-motivated candidate. If the axion exists, it must be extremely light, in the mass range of 10 Ϫ6-10 Ϫ3 eV, and possess extraordinarily feeble couplings to matter and radiation. Nevertheless, as proposed by Sikivie in 1983, the axion's two-photon coupling lends itself to a feasible search strategy with currently available technology. In this scheme, axions resonantly convert to single microwave photons by a Primakoff interaction, in a tunable microwave cavity permeated by a strong magnetic field. Present experiments utilizing heterostructure transistor microwave amplifiers have achieved total system noise temperatures of ϳ3 K and represent the world's quietest spectral radio receivers. Exclusion regions have already been published well into the band of realistic axion model couplings, within the lowest decade of mass range. Recent breakthroughs in the development of near-quantum-limited superconducting quantum interference device amplifiers should reduce the system noise temperature to ϳ100 mK or less. Ongoing research into using Rydberg-atom single-quantum detectors as the detector in a microwave cavity experiment could further reduce the effective noise temperature. In parallel with improvements in amplifier technology, promising concepts for higher-frequency cavity resonators are being explored to open up the higher decades in mass range. Definitive experiments to find or exclude the axion may therefore be at hand in the next few years. As the microwave cavity technique measures the total energy of the axion, a positive discovery could well reveal fine structure of the signal due to flows of nonthermalized axions. Manifesting diurnal and sidereal modulation, such detailed features would contain a wealth of information about the history, structure, and dynamics of our Milky Way galaxy. CONTENTS I. Overview 778 II. Review of Axion Theory 779 A. Particle physics and the axion 779 B. Constraints from laboratory searches and astrophysics 780 C. Axions and cosmology 781 D. Phase-space structure of halo dark-matter axions 783 III. The Sikivie Microwave Cavity Experiment 783 A. Principles and techniques 783 B. First-generation experiments 784 1. The Rochester-Brookhaven-Fermilab experiment 784 2. The University of Florida experiment 785 C. Second-generation experiments IV. The U.S. Large-Scale Search A. Hardware 1. Cavity and tuning rods 2. The cavity mode structure and form factor B. Balanced heterostructure field-effect transisto...

We report the first results of a high-sensitivity (∼ 10 −23 W) search for light halo axions through their conversion to microwave photons. At 90% confidence we exclude a KSVZ axion of mass 2.9 × 10 −6 eV to 3.3 × 10 −6 eV as the dark matter in the halo of our Galaxy.14.80. Mz, 95.35.+d, 98.35.Gi Typeset using REVT E X 3 The dynamics of galaxies and of clusters of galaxies, as well as their peculiar motions, imply that most of the mass of the Universe is in an unseen form, called 'dark matter'. The amount of dark matter inferred is at least 20% of the critical density, and likely much more [1]. Because the synthesis of the light elements in the big bang restricts baryons to contribute no more than 10% of the critical density, a large nonbaryonic component is required. The development of structure in the Universe -galaxies, clusters, and superclusters -and the anisotropies of the cosmic background radiation also support this conclusion.The axion is a well-motivated particle dark matter candidate, arising in models where the strong-CP problem is solved by the Peccei-Quinn mechanism [2]. The axion mass is constrained by laboratory experiments and astrophysical limits to lie between 10 −6 eV and 10 −3 eV, with lower masses preferred if axions provide the bulk of the critical density [3].If the dark matter is 'cold' (small velocity dispersion), as is indicated by studies of structure formation, galactic halos are comprised primarily of cold dark matter particles. Because dark matter axions were produced in a coherent process in the early universe, they are cold to be approximately Maxwellian, with a dispersion of β 2 1/2 270 km/sec [6]. There could also be narrow peaks in the velocity distribution from dark matter particles which have recently fallen into the galaxy and have yet to thermalize [7]. Because of its two-photon coupling, L aγγ = −g aγγ a E · B, an axion can convert to a single photon in the presence of a magnetic field [8]. Here g aγγ = g γ α/πf a , f a is the axion decay constant, the axion mass m a 6µeV (10 12 GeV/f a ), and g γ is a model-dependent coefficient of order unity. In two popular models of the axion, g γ = −0.97 (KSVZ) and 0.36 (DFSZ) [2].In a static magnetic field, the energy of the photon equals that of the converted axion:E γ = E a = m a +m a β 2 /2 = m a (1 +O(10 −6 )). The conversion process is resonantly enhanced in a high-Q cavity with resonant frequency f 0 tuned to E γ , with power given by [8] 4where V is the volume of the cavity, Q L is the loaded quality factor of the cavity, B 0 is the central magnetic field strength, ρ a is the local axion density, and 1/Q a ∼ 10 −6 is the width of the axion energy distribution. The mode-dependent form factor C is of order unity for the TM 010 mode used in our search and falls off rapidly for higher order modes. For the parameters of this experiment and the KSVZ model, P ∼ 5 × 10 −22 W.Because the axion mass is unknown, the cavity resonant frequency must be tuned. When the TM 010 resonant frequency is close to the axion mass, the conv...

We have built and operated a large-scale axion detector, based on a method originally proposed by Sikivie, to search for halo axions. The apparatus consists of a cylindrical tunable high-Q microwave cavity threaded axially by a static high magnetic field. This field stimulates axions that enter the cavity to convert into single microwave photons. The conversion is resonantly enhanced when the cavity resonant frequency is near the axion rest mass energy. The experiment is cooled to 1.5 K and the electromagnetic power spectrum emitted by the cavity is measured by an ultra-low-noise microwave receiver. The axion would be detected as excess power in a narrow line within the cavity resonance. The apparatus has achieved a power sensitivity better than 10 Ϫ23 W in the mass range 2.9-3.3 eV. For the first time the rf cavity technique has explored plausible axion models, assuming axions make up a significant fraction of the local halo density. The experiment continues to operate and will explore a large part of the mass in the range of 1 -10 eV in the near future. An upgrade of the experiment is planned with dc superconducting quantum interference device microwave amplifiers operating at a lower physical temperature. This next generation detector would be sensitive to even more weakly coupled axions contributing only fractionally to the local halo density.

The axion is a hypothetical elementary particle and cold dark matter candidate. In this RF cavity experiment, halo axions entering a resonant cavity immersed in a static magnetic field convert into microwave photons, with the resulting photons detected by a low-noise receiver. The ADMX Collaboration presents new limits on the axion-to-photon coupling and local axion dark matter halo mass density from a RF cavity axion search in the axion mass range 1.9-2.3 µeV, broadening the search range to 1.9-3.3 µeV. In addition, we report first results from an improved analysis technique.PACS numbers: 14.80. Mz, 95.35.+d, 98.35.Gi