The existence of axions was postulated in 1977 by Roberto Peccei and Helen Quinn, in a very famous paper with which they proposed an interesting solution to a very big open question of particle physics, known as strong CP problem:
The elementary forces have no reason to be invariant under CP, i.e., in general (and this is the case for the weak interaction), the way two particles interact will differ from the way their corresponding anti-particles interact. However, experiments have shown that the parameter governing the CP-violation for the strong interaction is extremely small. Since there is no theoretical justification for this tiny value in the Standard Model of particle physics, this 'naturalness problem', dubbed strong CP problem, has puzzled many physicists. The solution proposed by Peccei and Quinn adds an additional symmetry to the Standard Model and thus promotes the CP-violation parameter to a dynamical field, i.e. a particle (the axion), whose value is then dynamically fixed to be zero.
Nowadays, the existence of axions is still considered as the most plausible and theoretically attractive solution to the strong CP problem, and this conjecture is also supported by the fact that axions are currently one of the best candidates for Dark Matter. It has been estimated that if axions are actually there, they must have been produced in the early universe with great abundance but then they must have remained there to float in the empty space surrounding galaxies, being almost unable to interact with the rest of matter, and thus becoming invisible.
How do we hunt for axions, then? It is extremely important for physicists to figure out whether axions really exist and, if this is the case, to measure they properties. But can we do it? How? Well, one important thing to know is that there is not one unique way to plug the axion into the Standard Model of particle physics, but a rather large number of models have been proposed, which differ among each other which differ among each other in the axion physics they predict. Besides the original axions introduced by Peccei and Quinn to solve the strong CP problem, this concept has also been generalized to 'axion-like particles', ALPs. There is a variety of theoretical motivations for these, but most importantly, there is no reason to theoretically forbid such extra particles. Hence the only way to check if these particles are part of the zoo of elementary particles is to go look for them. All of these axion and ALP models share some common conclusions that can be exploited as a basis for experimental searches:
- that the axions must be very light, with masses smaller that an electronvolt (eV), i.e., more than one million times lighter than an electron
- that the axion must interact with ordinary matter, although very feebly. In other words, its coupling constant ought to be tiny, and in particular it should be inversely proportional to a characteristic energy scale, called fa, which therefore must be very large. The scale fa is an important parameter for understanding how axions behave, and it also gives us an idea of how early the axions were produced after the Big Bang (the larger fa the earlier the production).
- in particular, axions must interact with photons. As a consequence they must be produced in great quantities in stars, and they can be converted into X-rays (and vice-versa) when crossing an electric or magnetic field.
This latter point is a key one: the experiments searching for evidence of axion-photon conversion are those currently able to set the most stringent bounds on the axion-photon-photon vertex Ga-ph. The CERN Axion Solar Telescope (CAST) looks for axions emitted from the Sun, following our star during 1.5 h at sunrise and sunset, every day since 2003. It features a spare LHC superconducting magnet, able to create a monstrous 9 Tesla magnetic field over a length of 10m. Within such a field, solar axions are expected to be converted into X-rays of a specific wavelength, that would be focused and detected by a CCD camera. Since its kickoff, the experiment has crossed four operation phases, during which the conditions in which the detection system operated, and therefore the axion mass range to which it was sensitive, were changed. Not one single signal was observed in all this time, and this allowed to set an upper limit for the coupling Ga-ph:
Ga-ph < 3.3 10-10 GeV-1
for axion masses in the range
0.64 eV < ma < 1.12 eV
and (from an earlier operation phase)
Ga-ph < 8.8 10-11 GeV-1
if instead the axion mass is smaller than 0.02 eV. This looks very restrictive, but to get a realistic idea of the impact of this result, let's look at the following plot:
On the axes there are the axion mass (x-axis) and the coupling to photons (y-axis). The upper grey region is the one excluded by CAST, while the lighter grey bands provide the exclusion region identified by the ADMX experiment, which looks for galactic halo axions, trying to detect them by their resonant conversion into microwave signal inside an electromagnetic cavity permeated by a strong static magnetic field. The parameter space region corresponding to a solution of the strong CP-problem is the one marked by the yellow band: the green line highlights the prediction of one specific model, called KSVZ, while the orange regions are those for which axions can account for all the Dark Matter in the universe. As can be easily seen, the CAST limit is starting to probe the parameter space related to the strong CP-problem, but is not yet able to rule out a significant region of this theoretical prediction!
So more precision is needed in order to seriously challenge the yellow band. A very promising experiment in this sense is the International Axion Observatory (IAXO), which is being designed with the exact purpose of lowering the limits on solar axions. The technical design which has been proposed for IAXO should be able to push the sensitivity to Ga-ph down to 10-12 GeV-1 for masses up to 0.02 eV, thanks to the development of a dedicated magnet and to the optimization of the detector's geometry. As is shown in the plot above, this would provide a remarkable advance in the hunt for axions. Such a level of accuracy may even allow to exclude the existence of axion-like particles with very low masses, which have been proposed to explain anomalies in the propagation of light over large distances and are depicted by the red dashed line in the left-hand side of the plot.
In summary, this paper shows that helioscope axion searches have now reached exciting times. We are starting to probe the theoretically most motivated part of the parameter space, which much progress to be expected in the near future. At the same time, a range of further, complementary experiments (from astrophysical observations to earth-based searches) is dedicated to constraining the axion parameter space, joining forces in the hunt for this elusive particle.
Text by Ilaria Brivio and Valerie Domcke