New Ways to Search For Axion Dark Matter
Abstract: The QCD axion, and axion-like particles (ALPs), are some of the most theoretically well-motivated dark matter candidates. When these axions couple to photons they can be searched for with exceptional sensitivity in traditional cavity haloscope experiments. However, cavity haloscopes are limited in the masses of axions they can search for since the axion Compton wavelength must be matched to the size of the cavity. To search for high mass axions the detection channel must be decoupled from the detector size. Therefore in-medium excitations, e.g., electrons and phonons, are useful since they naturally have energies above a meV. We will discuss how axion absorption into in-medium excitations can be used to search for high mass axion dark matter when the target is immersed in an external magnetic field. These proposals synergize with parallel efforts in the direct detection community to use in-medium excitations to search for the scattering of other light dark matter candidates. In addition to the photon coupling, the axion may also couple directly to electrons. Searching for effects generated by this coupling requires dramatically different experiments. We will discuss how electron spin-ordered targets can be used to absorb axions into magnons, or layered to generate electromagnetic signals.