Physics (Internal)

Quest for the QCD Axion: DM Radio and Fundamental Limits of Electromagnetic Detection

This event is part of the Biophysics/Condensed Matter Seminar Series.

We discuss searches for the QCD axion, a well-motivated cold-dark matter candidate that also solves the strong CP problem in quantum chromodynamics. We focus on axion dark matter searches probing the coupling to electromagnetism and establish fundamental limits on detection subject to the Standard Quantum Limit on phase-insensitive amplification. These limits demonstrate that single-pole resonators are near-optimal single-moded detectors for axion dark matter. Furthermore, we show that proper optimization of measurement backaction and sensitivity outside of the resonator bandwidth can increase scan rates by up to five orders of magnitude at low frequency. We present DM Radio, a tunable, lumped-LC resonant search for axion and hidden-photon dark matter between 100 Hz and 300 MHz. Its design is informed by the fundamental limits and optimization statements. The prototype experiment, the DM Radio Pathfinder, uses a 4K liquid-helium-cooled detector to search for hidden photons in the 100 kHz-10 MHz frequency range. It is presently conducting engineering runs and narrowband measurements and will begin science scans in 2019. We discuss progress on the Pathfinder experiment, including investigations of resonator loss mechanisms and capacitive tuning, characterization of SQUID noise, and development of calibration and data analysis protocols. We present future plans and upgrades. In particular, we discuss a search using a detector volume of one cubic meter cooled to 10 mK and a several-Tesla magnet which will have sensitivity to the QCD axion above a few MHz.