Microwave Cavity Searches for Axions

The axion is a hypothetical elementary particle proposed as a solution to the ``strong CP'' problem. The mass of the axion is constrained by experimental and astrophysical considerations to a range where the axion is a very plausible cold dark matter candidate. This weakly interacting dark matter makes up the halo of our galaxy. In the ADMX experiment, halo axions flow through a microwave resonant cavity permeated by a static magnetic field where some convert into microwave photons. These photons are detected by an ultra-low-noise receiver. The ADMX Collaboration has set limits on the axion-to-photon coupling and /or local axion halo mass density for axion mass between 1.9 and 3.3 $\mu$eV.