Large-scale microwave cavity search for dark-matter axions - art. no. 092003

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 co nsists 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 resonant ly 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 spe ctrum emitted by the cavity is measured by an ultra-low-noise microwave rec eiver. 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(-23) W in the mass range 2.9-3.3 mu 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 continu es to operate and will explore a large part of the mass in the range of 1-1 0 mu 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 se nsitive to even more weakly coupled axions contributing only fractionally t o the local halo density.