Extended Search for Axion Dark Matter with ADMX

Thesis (Ph.D.)--University of Washington, 2021The axion is a hypothetical elementary particle that came out of a solution to the Strong-CP problem in Quantum Chromodynamics (QCD). The properties of the axion also make it a highly compelling candidate for dark matter. Axion haloscopes are instruments that search for axion dark matter in the local Milky Way halo by searching for the conversion of an axion into photons via the inverse Primakoff effect. The Axion Dark Matter eXperiment (ADMX) is by far the most sensitive axion haloscope, being the first to exclude benchmark models for the QCD axion and continuing to take data at high sensitivity. This thesis reports on a run by ADMX which excluded dark matter axions in the galactic halo over the mass range $2.81$--$3.31$ ${\mu}eV$. This unprecedented sensitivity in this mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first-stage signal amplifier