The axion was originally postulated to explain why the strong interactions conserve parity (P) and the product of parity with charge conjugation (CP). It has been shown that axions with mass in the neighborhood of 10-5 eV/c2 are the most abundant form of matter in the universe. They are, in fact, an excellent candidate for being the constituent matter of the dark-matter halos that envelop galaxies.
An experiment that attempts to detect Milky Way halo axions, called the Axion Dark Matter eXperiment, is aimed at observing the conversion of axions to microwave photons in an electromagnetic cavity which is tuned to the axion mass and which is permeated by a large static magnetic field. The experiment has set the first significant limits on the axion density for part of the interesting mass range.
Upgrades to ADMX have added SQUID amplifiers to the front-end electronics of the experiment and a dilution refrigerator to the cryogenic plant. Together, these upgrades yield noise temperatures in the 200 mK range. ADMX is sensitive to the most weakly coupled axion models and is beginning a search for axions in the 2 to 10 µeV range. Future upgrades, involving multiple cavities and/or higher magnetic fields will extend the search to 40 µeV. This search should either detect the axion or rule out the QCD axion as a significant component of the dark matter halo of our Galaxy.
For the latest details, see the ADMX axion page.
Papers about axion research.