Search for dark-matter axions
Prof. Dave Tanner
Distinguished Professor
Department of Physics
University of Florida
The nature of the dark matter in the Universe is one of the most compelling questions in all of science. Dark matter makes up roughly 85% of the Universe’s mass and we do not know what it is. It has gravity, but otherwise interacts extremely weakly with ordinary matter, making detection very challenging. The axion, a very well-motivated candidate for the dark matter, can be detected by conversion to microwave photons in a strong magnetic field. This process, invented by my colleague Pierre Sikive 40 years ago, is the basis of almost all axion searches. In the late 1980s, the Florida axion group built a detector to search for axions within the dark-matter halo of our Galaxy. The design of this experiment, including cavity configuration, materials, cavity tuning, data acquisition method, and data analysis approach was used by subsequent detectors, eventually becoming the Axion Dark Matter eXperiment (ADMX). The current “Generation 2” ADMX detector is sensitive to the high-priority models of axions as dark matter. The improvements allowing ADMX to reach this position were the incorporation of a high-performance dilution refrigerator and an ultrasensitive SQUID-based microwave amplifier, giving noise backgrounds in the 200 mK temperature range. The remaining components are a large-volume superconducting magnet and a high-Q tunable microwave cavity. The resulting limits on axion mass, the prospects for the ongoing search, and the outlook for the future will be discussed. I will also make an aside to discuss ADMX-SLIC, an LC circuit resonator that has the best sensitivity for searches for axions with mass below the lower limit of the ADMX cavity detector.