In my earlier posts, I mentioned several times that axions don’t interact very much with the particles in the standard model, with the exception of photons. Awesomely enough, not only can axions decay to photons, but photons can decay to axions as well! This is called the Primakoff Effect (and Inverse Primakoff Effect): when axions are in a strong magnetic field, they decay into photons (and vice versa, which is the Inverse Primakoff Effect). This opens up a big opportunity for experimental searches for axions – in principle, if axions exist, we can exploit the Primakoff Effect by using large magnets to induce magnetic fields that would – hopefully – lead axions to decay to photons and vice versa.
There are three types of axion experiments going on today – all of which are trying to exploit the Primakoff Effect: experiments using helioscopes (like CAST), some experiments using lasers (like PVLAS), and RF cavity experiments (like ADMX). Let’s take a closer look at each of these:
(1) ADMX. ADMX (The Axion Dark Matter eXperiment) is searching for axions converting into microwave photons. They use a huge 8 Tesla magnet in an RF cavity, and try to induce these axion-photon conversions and “tune” into (like tuning into a radio station) the frequency of the axions that have clustered around the halo of our galaxy (and in the Milk Way!).
(2) PVLAS. The PVLAS (Polarizzazione del Vuoto con LASer) experiment has been searching for axions since 2001. They use a gigantic polarized laser beam and a magnetic field to search for weird changes in the rotations of photon polarization. Due to the Primakoff effect, when axion-photon conversions occur, you should, in principle, be able to visibly detect changes in the photon polarization.
(3) CAST. Like ADMX and PVLAS, CAST (the CERN Axion Solar Telescope) is also searching for axion-photon conversions, but does so using a large telescope pointed at the Sun. They put a huge magnet near the telescope, and try to track axions that come from the Sun and convert them into photons!
Unfortunately, right now, there are no conclusive results from these experiments, though they have started singling out certain values that the axion mass can’t be – since they don’t see any axions at a certain mass, they can “exclude” these masses as possible values the axion mass could be.
To conclude my short summary of axions, I think that the discovery of axions would be nothing less than a victory for physics. If axions exist as described by Peccei-Quinn Theory, discovering them would mean a solution to the Strong CP Problem, which would be an amazing step forward for particle physics. If it happens to be the case that axions are the constituents of dark matter, their discovery would be revolutionary not only for particle physics, but for cosmology as well!