The Search for Axions

Pierre Sikivie (Florida)

The axion is a hypothetical particle proposed, approximately 40 years ago, to explain why the strong interactions are P and CP invariant. Additional motivation for its existence is due to the fact that a cold population
of axions is naturally produced in the early universe. These cold axions may constitute the dark matter today. I'll briefly review the limits on the axion from particle physics, stellar evolution and cosmology. The various constraints suggest that the axion mass is in the micro-eV to milli-eV range. In this range, its interactions
are so weak that the axion was once thought invisible. Nonetheless a number of methods have been proposed to search for so-called "invisible" axions. I'll describe these techniques, the experiments that have implemented them, and the results that have been obtained so far.