|Theorized||1977, Peccei and Quinn|
|Mass||10−6 to 1 eV/c2|
The axion is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.
Reasons for prediction
As shown by Gerardus 't Hooft, strong interactions of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits the violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by the weak interactions, the effective periodic strong CP violating term, Θ, appears as a Standard Model input parameter—its value is not predicted by the theory, but must be measured. However, large CP violating interactions originating from QCD would induce a large electric dipole moment for the neutron. (While the neutron is an electrically neutral particle, nothing prevents charge separation within the neutron itself.) Experimental constraints on the currently unobserved neutron's electric dipole moment imply that CP violation arising from QCD must be extremely tiny and thus Θ must itself be extremely small or absent. Since a priori Θ could have any value between 0 and 2π, this presents a naturalness problem for the standard model. Why should this parameter find itself so close to 0? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem.
One simple solution exists: if at least one of the quarks of the standard model is massless, Θ becomes unobservable; i.e. it vanishes from the theory. However, empirical evidence strongly suggests that none of the quarks are massless and so the strong CP problem persists.
In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei–Quinn mechanism. The idea is to effectively promote Θ to a field (particle). This is accomplished by adding a new global symmetry (called a Peccei–Quinn symmetry) to the standard model that becomes spontaneously broken. Once this new global symmetry breaks, a new particle results and, as shown by Frank Wilczek and Steven Weinberg, this particle fills the role of Θ—naturally relaxing the CP violation parameter to zero. This hypothesized new particle is called the axion. (On a more technical note, the axion is the would-be Nambu–Goldstone boson that results from the spontaneously broken Peccei–Quinn symmetry. However, the non-trivial QCD vacuum effects (e.g. instantons) spoil the Peccei–Quinn symmetry explicitly and provide a small mass for the axion. Hence, the axion is actually a pseudo-Nambu–Goldstone boson.) The original Weinberg-Wilczek axion was ruled out by data. In the current literature the axion mechanism is discussed in the form of the invisible axion which exists in two versions: the KSVZ axion  and the DFSZ axion.
It had been thought that the invisible axion provides a solution to the strong CP problem without being amenable to verification by experiment or observation. In invisible axion models, the axion is chosen to be so weakly coupled that it does not appear in any of the experiments that had been attempted to detect it. The very weakly coupled axion is also very light because the axion couplings and axion mass are proportional to each other. The situation changed when it was shown that a very light axion is overproduced in the early universe and therefore excluded.  The axion can not be arbitrarily weakly coupled therefore. The cricital mass is of order 10−11 times the electron mass, with large uncertainties. When the axion mass is of that order, axions may account for the dark matter in the universe. The axion is thus a dark matter candidate as well as a solution to the strong CP problem. Furthermore, in 1983, Pierre Sikivie wrote down the modification of Maxwell's equations that results from the existence of a light long-lived axion  and using these equations showed that dark matter axions can be detected on Earth by converting them to microwave photons in an electromagnetic cavity tuned to the axion mass and permeated by a strong magnetic field. This is the principle of the Axion Dark Matter eXperiment (ADMX) at the University of Washington in Seattle. He also showed that it is possible to search for axions emitted by the Sun by converting them to x-rays in a laboratory magnetic field, the principle of the CERN Axion Solar Telescope (CAST).
A number of experiments have attempted to detect axions, including at least one that has claimed positive results.
In the Italian PVLAS experiment polarized light propagates through the magnetic field of a 5 T dipole magnet, searching for a small anomalous rotation of the direction of polarization. The concept of the experiment was first put forward in 1986 by Luciano Maiani, Roberto Petronzio and Emilio Zavattini, and if axions exist, photons could interact with the field to become virtual or real axions. This rotation is very small and difficult to detect, but this problem can be overcome by reflecting light back and forth through the magnetic field millions of times. The most recent PVLAS results do detect an anomalous rotation, which can be interpreted in terms of an axion of mass 1–1.5 meV. However, there are other possible sources for such an effect besides axions.
Several experiments search for axions of astrophysical origin using the Primakoff effect. This effect causes conversions of axions to photons and vice versa in strong electromagnetic fields. Axions can be produced in the Sun's core when x-rays scatter off electrons and protons in the presence of strong electric fields and are converted to axions. The CAST experiment is currently underway to detect these axions by converting them back to x-rays in a strong magnetic field.
The Axion Dark Matter Experiment (ADMX) searches for light, weakly interacting axions saturating the dark matter halo of our galaxy. ADMX is a strong magnetic field permeating a cold microwave cavity. Axions matching the resonant frequency of the cavity decay into microwave photons. ADMX has excluded optimistic axion models in the 1.9 μeV to 3.53 μeV range. The microwave cavity experiment known as ADMX in 1996–2010 failed to detect axions having a mass range of 1.98–2.17 µeV and a frequency between 450 and 850 MHz. ADMX is amidst a series of upgrades and is currently taking data in new mass and coupling ranges.
Another means of searching for axions is by conducting so called "light shining through walls" experiments, where a beam of light is passed through an intense magnetic field in an attempt to observe the conversion of photons into axions by allowing them to pass through an aluminium plate, blocking the passage of photons. However, these practices are of low efficacy, necessitate high initial photon flux, and those conducted by BFRS and PVLAS have been the subject of some further verification. A recent experiment had the necessary sensitivity to detect this effect if the PVLAS 2005-signal was due to axions; however, no effect was seen.
On 9 July 2007, a paper submitted to arXiv by Carlo Rizzo and other researchers from the Centre National de la Recherche Scientifique indicated with a confidence level of 94% or higher, that they believed the results published by the PVLAS experiment, in Italy, were incorrect and did not prove the existence of the axion. Initially, the team researched the matter after their claim that the axion coupling inferred from the PVLAS experiment did not match with experiments conducted in 2007 and earlier in 2006, and thus required review.
The experiment conducted by Rizzo's team differed from the approach of the Italian researchers in the fact that at the end of a vacuum chamber, an aluminium plate was placed to prevent photons from an adjacent laser from passing through the plate, where axions would simply pass through the plate and be converted back into photons, and were able to observe a small-portion of the supposed-converting particles—to the number of 4×1022 photons.
In the use of optical measurement and pulsating beams of light, the team showed through illustration of exclusion curves compared to the PVLAS experiment and another conducted by the BFRT, that the axion had been ruled out but still remained a valid hypothesis; the experiment counting as an important step in the understanding of the particle, with the possibility of a very weak coupled axion.
A few days earlier, on the 23 June, the PVLAS had submitted a paper to arXiv, in which they noted that upgrades to their measurement systems had been undertaken to increase the accuracy of their results from the previous year, through the use of 2.3 and 5.5 T fields and wavelengths of 1064 nm. With this increased accuracy, PVLAS had noted that the axion particle interpretation had been ruled out due to the absence of a rotational signal on the levels of 1.2×10−8 rad×5.5 T and 1.0×10−8 rad×2.3 T with 45000 passes.
Resonance effects of axions may be evident in Josephson junction devices. A supposed high flux of axions from the galactic halo with a mass of 0.11 meV appears to create excess electrical current in a certain type of Josephson junction.
Axions and other light bosons are also expected to have an observable signature in various astrophysical settings. In particular, several recent works have proposed the existence of axion-like particles as a possible solution to the apparent transparency of the Universe to TeV gamma-ray radiation. It has also been demonstrated in a few recent works that, on account of the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., magnetars), such environments will convert photons to axions much more efficiently than most laboratory experiments, over a broad axion mass range. This, in turn, would give rise to distinct absorption-like features in the spectra of such objects which can be observed by current telescopes and, therefore, significantly increase our sensitivity to axion detection. A new promising detection means for axions and axion-like particles is by looking for quasi-particle beam refraction effects in systems with strong magnetic field gradients such as radio-loud magnetars and magnetic pulsars. In particular, the refraction effects will lead to beam splitting effects that can be easily detected in the radio light curves of highly magnetized pulsars and could allow for the detection of light bosons with much greater sensitivities than are currently achievable by other means.
Axions in condensed-matter physics
A term analogous to the one that must be added to Maxwell's equations also appears in recent theoretical models for topological insulators. This term leads to several interesting predicted properties at the interface between topological and normal insulators. In this situation the field θ describes something very different from its use in high-energy physics. In 2013, Christian Beck suggested that axions might be detectable in Josephson junctions; and in 2014, he argued that an axion signature, consistent with a mass around 110μeV, had in fact been observed in several preexisting experiments.
One theory of axions relevant to cosmology had predicted that they would have no electric charge, a very small mass in the range from 10−6 to 1 eV/c2, and very low interaction cross-sections for strong and weak forces. Because of their properties, axions interact only minimally with ordinary matter. Axions are predicted to change to and from photons in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions.
In supersymmetric theories the axion has both a scalar and a fermionic superpartner. The fermionic superpartner of the axion is called the axino, the scalar superpartner is called the saxion. In some models, the saxion is the dilaton. They are all bundled up in a chiral superfield.
Theory[which?] suggests that axions were created abundantly during the Big Bang. Because of a unique coupling to the instanton field of the primordial universe (the "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy.
If axions have low mass, thus preventing other decay modes, axion theories predict that the universe would be filled with a very cold Bose–Einstein condensate of primordial axions. Hence, depending on their mass, axions could plausibly explain the dark matter problem of physical cosmology. Observational studies to detect dark matter axions are underway, but they are not yet sufficiently sensitive to probe the mass regions where axions would be expected to be found if they are the solution to the dark matter problem. High mass axions of the kind searched for by Jain and Singh (2007) would not persist in the modern universe and could not contribute to dark matter. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.
Low mass axions could have additional structure at the galactic scale. As they continuously fell into a galaxy from the intergalactic medium, they would be denser in "caustic" rings, just as the stream of water in a continuously-flowing fountain is thicker at its peak. The gravitational effects of these rings on galactic structure and rotation might then be observable. Other cold dark matter theoretical candidates, such as WIMPs and MACHOs, could also form such rings, but, because such candidate particles or objects are fermionic and thus experience friction and/or scattering among themselves, the rings would be less pronounced than with bosons such as the axion.
Axions, if they exist, would also have stopped most interaction with normal matter at a different moment in the big bang than other more massive particles hypothesized for dark matter. The lingering effects of this difference could perhaps be calculated and observed astronomically.
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- CAST Experiment
- CAST at MPI/MPE
- CAST at University of Technology Darmstadt
- ADMX at University of Washington