Laboratory searches for GUT-scale axion dark matter beyond the magnetoquasistatic limit

In the coming years laboratory experiments such as ABRACADABRA and DMRadio will cover some of the most well-motivated territory for axion dark matter. In particular, they will probe QCD axion masses and photon couplings that could be associated with axions produced at the scale of grand unification. However, to-date the sensitivity projections for these experiments have relied on the magnetoquasistatic approximation, which assumes that the quantum wavelength of the axion is much larger than the size of the experiment. Here, we present first results for understanding the response of ABRACADABRA and DMRadio type axion detectors beyond the magnetoquasistatic limit. We show that the sensitivity of these experiments degrades rapidly when the Compton wavelength is smaller than the detector size, but we also present possible novel detector modifications that could extend the sensitivity of the detectors to higher masses.