Axion searches

The strong CP problem and its resolution through the existence of an axion are briefly reviewed. The constraints on the axion from accelerator searche s, from the evolution of red giants and from supernova SN1987a combine to r equire m(a) < 3 .10(-3) eV, where m(a) is the axion mass. On the other hand , the constraint that axions do not overdose the universe implies m(a) grea ter than or similar to 10(-6) eV. If m(a) similar to 10(-5) eV, axions cont ribute significantly to the cosmological energy density in the form of cold dark matter. Dark matter axions can be detected by resonant conversion to microwave photons in a cavity permeated by a static magnetic field and tune d to the axion mass. Experiments using this effect are described, as well a s several other types of axion searches.