Panofsky Prize Presentation—The search for the invisible axion

Forty years ago, Pierre Sikivie came to my lab at Florida, where I studied far-infrared/millimeter wave effects in solids, to tell me about an experiment that he had invented, the cavity haloscope. I joined Pierre and Neil Sullivan, a UF NMR experimentalist, to discuss the practical aspects of implementing this concept. With equipment from our labs, with considerable support from the University sponsored program office, and with Florida graduate student Chris Hagmann, the pilot axion haloscope was built and operated at UF. It used an 8 Tesla magnet, a tunable 1.5 GHz copper resonant cavity, and a cryogenic microwave amplifier, setting in 1990 limits on the coupling of the axion to two photons about 100x above the level predicted by high-priority models of the axion. The Florida group is now part of the Axion Dark Matter eXperiment (ADMX) which has in the last few years demonstrated the needed sensitivity. ADMX data are analyzed within two channels: Medium and High Resolution (HiRes), with respective frequency resolutions of 200 Hz and as low as 20 mHz. The HiRes channel is sensitive to flows of non-virialized axions that may exist within the Galactic halo. These cold flows are expected to have very low velocity dispersion, leading to correspondingly narrow peaks in the measured power spectra. Analysis gives limits on the axion density in such flows at spectral resolutions of 20, 40, 200, and 1000 mHz. Prospects for the future include the use of multiple cavities and a larger and stronger magnet located at Fermilab.