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The axion is a hypothetical elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

 postulated by the Peccei-Quinn theory
Peccei-Quinn theory
In particle physics, the Peccei–Quinn theory is the best known proposal for the resolution of the strong CP problem. The theory proposes that the QCD Lagrangian be extended with a CP-violating term known as the θ parameter...

 in 1977 to resolve the strong CP problem in quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...

 (QCD). If axions exist and have low mass within a certain range, they are of interest as a possible component of cold dark matter
Cold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...

.

Reasons for prediction


As shown by Gerardus 't Hooft
Gerardus 't Hooft
Gerardus 't Hooft is a Dutch theoretical physicist and professor at Utrecht University, the Netherlands. He shared the 1999 Nobel Prize in Physics with his thesis advisor Martinus J. G...

, strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...

s of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits the violation of the combined symmetries of charge conjugation
C-symmetry
In physics, C-symmetry means the symmetry of physical laws under a charge-conjugation transformation. Electromagnetism, gravity and the strong interaction all obey C-symmetry, but weak interactions violate C-symmetry.-Charge reversal in electromagnetism:...

 and parity
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...

, collectively known as CP. Together with effects generated by the weak interaction
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

s, the effective strong CP violating term, , appears as a Standard Model input parameter—it 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
Neutron electric dipole moment
The neutron electric dipole moment is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has...

. (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 must itself be extremely small or absent. Since a priori could have any value between 0 and 2π (the parameter is periodic), 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, 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
Roberto Peccei
Roberto Daniele Peccei is Vice Chancellor for Research at the University of California Los Angeles, a position he has held since October, 2000...

 and Helen Quinn
Helen Quinn
Helen Rhoda Quinn is an Australian-born particle physicist. She went to school in Victoria, Australia and entered college at the University of Melbourne before moving to the USA and transferring to Stanford University. She received her Ph.D. from Stanford in 1967, at a time when less than 2% of...

 postulated a more elegant solution to the strong CP problem, the Peccei-Quinn mechanism
Peccei-Quinn theory
In particle physics, the Peccei–Quinn theory is the best known proposal for the resolution of the strong CP problem. The theory proposes that the QCD Lagrangian be extended with a CP-violating term known as the θ parameter...

. The idea is to effectively promote 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 —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 (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.)

Experimental searches


A number of experiments have attempted to detect axions, including at least one that has claimed positive results.

In the Italian PVLAS
PVLAS
PVLAS is an apparatus dedicated to the detection of dark matter. It is located at the Legnaro National Laboratory of National Institute of Nuclear Physics near Padova, Italy...

 experiment polarized light propagates through the magnetic field of a 5 T
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

 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
Luciano Maiani
Luciano Maiani is a San Marino citizen physicist best known for his prediction of the charm quark with Sheldon Lee Glashow and John Iliopoulos .-Academic history:...

, Roberto Petronzio and Emilio Zavattini
Emilio Zavattini
Emilio Zavattini was an Italian physicist.-Biography:He was born in Rimini, Italy and enrolled in the University of Rome La Sapienza as a physics student in 1950 and earned his doctorate in 1954....

, and if axions exist, photons could interact with the field to become virtual or real axions. This rotation is very, 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–. However, there are other possible sources for such an effect besides axions.

Several experiments search for axions of astrophysical origin using the Primakoff effect
Primakoff effect
Primakoff effect is the resonant production of neutral pseudoscalar mesons by high-energy photons interacting with an atomic nucleus...

. 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 gamma rays in a strong magnetic field.

The Axion Dark Matter Experiment (ADMX)
Axion Dark Matter Experiment
The Axion Dark Matter eXperiment is an ongoing experimental search for axions in our galaxy's dark matter halo.-Background:...

 searches for light, weakly interacting axions saturating the dark matter halo
Dark matter halo
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

 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.

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
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 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 PLVAS 2005-signal was due to axions; however, no effect was seen.

On 9 July 2007, a paper submitted to arXiv
ArXiv
The arXiv |Chi]], χ) is an archive for electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance which can be accessed online. In many fields of mathematics and physics, almost all...

 by Carlo Rizzo and other researchers from the Centre National de la Recherche Scientifique
Centre national de la recherche scientifique
The National Center of Scientific Research is the largest governmental research organization in France and the largest fundamental science agency in Europe....

 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 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
PVLAS
PVLAS is an apparatus dedicated to the detection of dark matter. It is located at the Legnaro National Laboratory of National Institute of Nuclear Physics near Padova, Italy...

 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
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

 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  ×  and  ×  with passes.

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 accounts of the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., Magnetar
Magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious high-energy electromagnetic radiation, particularly X-rays and gamma rays...

s), 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.

A pair of papers by Katherine Mack that appeared on the arXiv in November 2009 casts doubt on the existence of the QCD
QCD
The initialism QCD may refer to:* Quantum chromodynamics, the theory describing the Strong Interaction* Quantum circuit diagram* Quality, Cost, Delivery, a three-letter acronym used in lean manufacturing...

 axion, arguing that (1) cosmological observations imply that the existence of the axion creates a greater fine-tuning problem than the one that it was hypothesized to solve and (2) that anthropic arguments do not alleviate this fine-tuning problem. These papers argue that, since the very motivation for the axion was to fix a fine-tuning problem, but its existence creates a different, and larger, fine-tuning problem—and, moreover, since the axion has never been observed—there is no longer any good reason to suspect that they exist.

Axions in condensed matter physics


A term analogous to the one that must be added to Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.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.

Predictions


One theory of axions relevant to cosmology
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

 had predicted that they would have no electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

, a very small mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 in the range from 10−6 to , and very low interaction cross-sections
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....

 for strong
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...

 and weak
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

 forces. Because of their properties, axions interact only minimally with ordinary matter. Axions are predicted to change to and from photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions.

Supersymmetry


In supersymmetric theories
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...

 the axion has both a scalar and a fermionic superpartner
Superpartner
In particle physics, a superpartner is a hypothetical elementary particle. Supersymmetry is one of the synergistic theories in current high-energy physics which predicts the existence of these "shadow" particles....

. The fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....

ic superpartner of the axion is called the axino
Axino
The axino is a hypothetical elementary particle predicted by some theories of particle physics. Peccei-Quinn theory attempts to explain the observed phenomenon known as the strong CP problem by introducing a hypothetical real scalar particle called the axion...

, the scalar superpartner is called the saxion. In some models, the saxion is the dilaton
Dilaton
In particle physics, a dilaton is a hypothetical particle. It also appears in Kaluza-Klein theory's compactifications of extra dimensions when the volume of the compactified dimensions vary....

.
They are all bundled up in a chiral superfield
Chiral superfield
In theoretical physics, one often analyzes theories with supersymmetry in which chiral superfields play an important role. In four dimensions, the minimal N=1 supersymmetry may be written using the notion of superspace...

.

The axino has been predicted to be the lightest supersymmetric particle
Lightest Supersymmetric Particle
In particle physics, the Lightest Supersymmetric Particle is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models. In models with R-parity conservation, the LSP is stable. There is extensive observational evidence for an additional...

 in such a model. In part due to this property, it is considered a candidate for the composition of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

.

Cosmological implications


Theory suggests that axions were created abundantly during the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

. Because of a unique coupling to the instanton
Instanton
An instanton is a notion appearing in theoretical and mathematical physics. Mathematically, a Yang–Mills instanton is a self-dual or anti-self-dual connection in a principal bundle over a four-dimensional Riemannian manifold that plays the role of physical space-time in non-abelian gauge theory...

 field of the primordial universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 (the "misalignment mechanism
Misalignment mechanism
The misalignment mechanism is a hypothesized effect in the Peccei-Quinn proposed solution to the strong-CP problem in quantum mechanics. The effect occurs when a particle's field has an initial value that is not at or near a potential minimum...

"), an effective dynamical friction
Dynamical friction
Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space...

 is created during the acquisition of mass following cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...

. 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
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 problem of physical cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

. 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. 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. 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.

Low mass axions could have additional structure at the galactic scale. As they continuously fell into a galaxy from the intergalactic medium, they might be denser in "caustic
Caustic (mathematics)
In differential geometry and geometric optics, a caustic is the envelope of rays either reflected or refracted by a manifold. It is related to the optical concept of caustics...

" rings. 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 MACHO
Macho
Macho typically refers to machismo. Other uses include:*Macho , a short-lived disco group in the late 1970s*Pique macho, Bolivian dish*Macho Man , a 1978 disco song performed by the Village People...

s, would also form such rings, but, because such candidates are fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....

ic, the rings would be less pronounced than with bosons such as the axion.

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