QCD, the theory of strong interactions allows the violation of CP symmetry that would result in a non-zero value for the neutron electric dipole moment. Current measurements constrain the neutron EDM to be one trillionth of the size predicted by QCD (because of CP violation). This means that the strength of this interaction (parameterized as an angle θ) has to be fine-tuned to an extremely small value of less than 10⁻¹⁰ rad. This constitutes a fine-tuning problem that challenged theoretical physics back in the ‘70s.

The axion, or more specifically a new field that could be responsible for the small values of θ, was introduced to solve this problem. In 1977, Robert Peccei and Helen Quinn developed a theory which promoted the angle θ from a fixed parameter to a field that has an additional chiral symmetry. In analogy to the Higgs mechanism, the spontaneous breaking of this symmetry gives rise to a new field which was later called the axion field. The axion was identified by Weinberg and Wilczek as the pseudo Nambu-Goldstone (pNG) boson of the new spontaneously broken global symmetry that Peccei and Quinn had postulated. Adding the axion field in the QCD Lagrangian can solve the neutron EDM problem. In this issue, Peccei and Quinn recall the interesting developments that led to the formulation of the Peccei-Quinn mechanism and reflect on future searches for axions and physics beyond the Standard Model.

Axion–photon coupling versus the rest mass of QCD-inspired axions, showing the limits achieved by the helioscopes RBF, Sumico and CAST in the context of axion searches in general. It also shows astrophysically and cosmologically derived conclusions from HB stars and the hot dark matter (HDM) limit (Imace Credit: CERN Courier). 

Recently, interest in axion searches has heightened as axions of a specific mass range are good candidates of cold dark matter. Many experiments around the world have launched extensive DM axion searches to cover the relevant parameter space. Evidence for cold dark matter was first obtained from considering the rotation curves of outer objects in galaxies, which reveal that the average galactic mass not only consists of dust and gases, but also and for its most part of a nonluminous halo of unknown composition that doesn’t emit nor absorb light at any significant level. The majority of the dark matter observed has the form of Cold Dark Matter, which means that this exotic component was nonrelativistic at the time of galaxy formation, thus pointing to searches for non-relativistic axions.

Existing theoretical underpinnings of the axion do not predict a value for the axion mass. Cosmological considerations indicate that if the axion is the source of the cold dark matter in the universe, it should have a mass in the range of about 1–25 μeV∕c² - about 10¹⁵ times smaller than the WIMP’s. With such a small mass, we would expect tens of trillions of axions per cubic centimeter in our Solar System to account for the observed dark matter density. However, the interactions of axions with ordinary matter and photons are expected to be so feeble  - they could fly through the whole Universe even if it was made out of bricks - that their detection requires extremely sensitive techniques.

Axions that could solve both the strong CP and DM problem, should have very specific mass ranges (very light) and be copiously produced. The phenomenology of axions and Axion-like particles is determined by their low mass and very weak interactions and they could affect stellar evolution and cosmology in a similar way to thermal neutrinos. The produced axions are non-relativistic particles and their contribution to the dark matter density scales as Ωaxion ∼ m −7/6 a . This scaling unambiguously implies that the axion mass must be fine-tuned ma = 10⁻⁵ eV to saturate the DM density today while larger axion mass will contribute very little to ΩDM. Much lighter axions would overclose the universe, and therefore ma ≈ 1 µeV may be taken as a strong lower limit on the axion mass. Moreover, stellar evolution also places strict limits on the upper mass of axions.  Axions with masses larger than ∼ 16 meV would have quenched the neutrino pulse observed from SN1987a, thus bounding the axion mass from above.

In this issue, we interview Pierre Sikivie, who first came up with ideas on how to detect this seemingly invisible particle. The technique relies on the fact that the axion strongly couples to the photon when travelling inside a magnetic field. This permits the conversion of an axion or ALP into a single real photon in an external electromagnetic field, i.e. the so-called Primakoff effect, as well as the inverse process. Axions can be also produced as a result of the Primakoff effect in a stellar plasma at high temperature and in that case we have ultra-relativistic axions. Axions travelling through a magnetic field can convert to photons and the probability of this transformation depends on the intensity of the magnetic field.

This fact motivated the searches of axions with dedicated resonating (RF) cavities. The power of this method depends on the axion mass, its coupling constant to photons, their local density as well as on the volume, the form factor and the loaded quality factor of the cavity. The key idea is that one the axions will fly through the cavity, where it will interact with its magnetic field and convert to a photon of a specific wavelength. For the conversion to take place, the frequency of the cavity must equal the mass of the axion resulting in an extremely monochromatic signal. The search is performed by tuning the cavity in small overlapping steps and the expected signal power is extraordinarily tiny.

Deriving from Sikivie’s ideas comes two other devices: the axion helioscope that could detect the copious flux of axions emitted from the Sun and the axion haloscope to detect axions from the hypothetical dark matter galactic halo. Sikivie’s proposal demonstrated that we can use natural sources of axions which are extremely efficient, like the Sun or even relic axions from the Big Bang. Because of the extremely large fluxes of natural axions, helioscopes and haloscopes are typically much more sensitive to axions and ALPs than their purely laboratory competitors, although their luminosities are also subject to larger uncertainties, especially in the case of dark matter axions.

CERN’s diverse experimental programme include searches for axions and ALPs with a number of different experimental technologies. In this special issue we present results of direct searches at the LHC through proton and heavy-ion collisions as well as in-depth report from the dedicated haloscope (CAST) with its proposed upgrade and a discussion of the most recent OSQAR results. 

During the last two decades, the efforts and size of the community have been steadily growing but the few last years have witnessed a real blossoming of ideas. Many new groups have entered the field, new exciting detection concepts have been proposed and several demonstrative small-scale setups have been commissioned.

The author would like to thank Dr. Babette Dobrich (CERN) for her thoughtful comments in the preparation of these articles.