Axion Dark Matter

Axions, hypothetical particles proposed to solve the strong CP in the Peccei-Quinn mechanism, are amongst the best theoretically motivated candidates for the dark matter of our universe. Despite this fact, until very recently only few experiments were actually planning to look for them. This contrast with the overwhelming abundance of nuclear-recoil experiments looking for weakly-interacting massive particles (WIMPs), which despite huge efforts have yet not found convincing evidence of their existence. This fact, together with the non-observation of supersymmetric (SUSY) particles at the large hadron collider (LHC) and the revival of the low-energy frontier of particle physics is triggering the rising interest of searching for alternative dark matter candidates, and here axions are top of the list.

Axions are ultralight particles (mass smaller than a meV) which interact very feebly with ordinary matter and photons. Therefore, axion dark matter experiments could not be more different than WIMP searches. The most promising technique is searching for tiny narrowband signals in microwave cavities immersed in strong magnetic fields. Here tiny means powers smaller than a yoctoWatt, which require high-Q cavity power enhancements and cryogenic detectors. Unfortunately, the enhancement in detection capabilities only holds for a very small range of axion masses (typically 1/Q) so many different experiments are needed to cover the huge range of axion masses where they could be dark matter (shown as red bands in the above figure). A combined international effort is required to scan over all possibilities in a reasonable amount of time.

The ADMX experiment in Seattle aims at covering the range 2-8 microeV in mass in 2015. The CARRACK experiment in Tokyo was designed to be extemely sensitive in the 10 microeV range. An offspring of the ADMX collaboration in Yale (ADMX-HF) is working on new ideas to cover the 10-20 microeV range. It appears that nowadays there are no viable experiments capable of detecting dark matter axions beyond this mass. Although there some ideas with dish-antennas and long cavities that we will discuss in the workshop.

In this high mass range, the very slow decay of axion DM into two photons is not that slow and one could search for the consequent narrow emission line in DM rich (and MW background poor) regions of the universe.

If the axion mass is below 2 microeV, the required MW cavity is quite a volume. There are no plans to build large resonant cavities in the required large magnetic fields. The future International AXion Observatory (IAXO) has been proposed to search for solar axions with an unprecedent sensitivity. It will require a huge dedicated toroidal magnet, similar to the one used by the ATLAS experiment with an impressive magnetic volume available for axion DM searches.

In the nanoeV mass range, new techniques have been very recpntly roposed to search for axion DM. They involve precision magnetometrey and resonant LC circuits. In this mass range, axion DM tends to be too abundant, requiring specific cosmological modeols to fit the observations. Axion models predicted in Grand Unified theories and String theories predict axions in this mass range.