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.

Most of the mass of the Milky Way is contributed by its halo, presumably in the form of noninteracting cold dark matter. The axion is a compelling cold dark matter candidate. We report results from a search that probes the local Galactic halo axion density using the Sikivie radio frequency cavity technique. Candidates over the frequency range 550 MH MHz (2.3 me meV) were investigated. The absence of a signalsuggests that the axions of Kim and Shifman, Vainshtein, & Zakharov contribute no more than 0.45 GeV cm Ϫ3 of mass density to the local dark matter halo over this mass range.

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We report first results from a large-scale search for dark matter axions. The experiment probes axion masses of 1.3-13 µ eV at a sensitivity which is about 50 times higher than previous pilot experiments. We have already scanned part of this mass range at a sensitivity better than required to see at least one generic axion model, the KSVZ axion. Data taking at full sensitivity commenced in February 1996 and scanning the proposed mass range will require three years.