The axion is a hypothetical elementary particle that would solve two major problems in science. It was originally a consequence of a solution to explain the lack of CP-violation in the physics of quarks and gluons, the fundamental constituents of objects such as protons and neutrons. Later, it was recognized to be a good dark matter candidate. Today, it remains as one of the leading dark matter candidates.
On the left is a particularly useful decay channel of the axion. The axion decays into two photons, and (as it turns out) give us the best chance of axion detection. On the right is a well-studied spiral galaxy (here NGC 3198), thought to consist mostly of cold dark matter. Our experiment addresses both the physics of the very small and physics of the very large.
By definition, the axion interacts only very weakly with ordinary matter and is difficult to detect. Nevertheless, Pierre Sikivie devised a method to find these axions via conversion into photons within a microwave cavity threaded with a static magnetic field. Our experiment is a large scale implementation of his idea. We have been in production since 1997 in search of this elusive particle ...
To learn more about axion physics, see Pierre's very accessible The Pool Table Analogy to Axion Physics that appeared in Physics Today. A popular science account of our experiment can be found in Lawrence Livermore's Science & Technology Review.