Weighing the Axion with Muon Haloscopy

The axion was introduced over forty years ago to explain the apparent symmetry of the strong nuclear force under time-reversal T and space inversion P. Later, it became a leading dark matter candidate when laboratory searches indicated its decay constant is large compared to the electroweak symmetry breaking scale. Yet, the most sensitive direct searches for axions in the local dark matter halo of the galaxy using radio-frequency electromagnetic cavities have yielded no positive results. In this talk we argue that measurements of the muon magnetic moment at the level of precision currently achieved using muon beams in storage rings can directly detect such axions in a couple runs provided the axion mass lies in the near-infrared regime and the beam cross-section can be increased to 10 cm² while keeping fixed the number of muons in the beam. We give a symmetry argument that leads to a mass in this range provided axions form the bulk of the dark matter in the observable universe. Finally, we show that axions saturate the dark matter density in the local halo by evaluating their contribution to the T and P breaking decay of the long-lived neutral K meson into a pair of charged pions.