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Axion

(Redirected from Axions)
Not to be confused with axiom or axon. For other uses, see Axion (disambiguation).

An axion (/ˈæksiɒn/) is an elementary particle whose existence was deduced independently by Frank Wilczek and Steven Weinberg in 1978 using the Peccei–Quinn theory that resolves the strong CP problem raised by quantum chromodynamics (QCD). There are six axions for every photon made in the Big Bang. They form the dark matter in the universe.

Axion
CompositionNormal antimatter and mirror matter
InteractionsGravitational, electromagnetic, strong nuclear, weak nuclear
StatusObserved
SymbolA0, a, θ
Theorized1978, Wilczek and Weinberg
Mass 0.50580(2)eV/c2 [1]
Electric charge0
Spin0

History

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Strong CP problem

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As shown by Gerard 't Hooft,[2] strong interactions of the standard model, QCD, possess a non-trivial vacuum structure[a] that in principle permits violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by weak interactions, the effective periodic strong CP-violating term, Θ, appears as a Standard Model input – 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 (EDM) for the neutron. Experimental constraints on the unobserved EDM implies CP violation from QCD must be extremely tiny and thus Θ must itself be extremely small. Since 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 zero? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem.[b]

Prediction

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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. This is accomplished by adding a new global symmetry (called a Peccei–Quinn (PQ) symmetry) that becomes spontaneously broken. This results in a new particle, as shown independently by Frank Wilczek[5] and Steven Weinberg,[6] that fills the role of Θ, naturally relaxing the CP-violation parameter to zero. Wilczek named this new hypothesized particle the "axion" after a brand of laundry detergent because it "cleaned up" a problem,[7][8] while Weinberg called it "the higglet". Weinberg later agreed to adopt Wilczek's name for the particle.[8] Because it has a non-zero mass, the axion is a pseudo-Nambu–Goldstone boson.[9]

Observation

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Axions from the local dark matter halo of the galaxy couple to the virtual photon that dresses the electromagnetic vertex of the muon.[1] The coupling of the halo axions to the virtual photon shifts the effective value of the electromagnetic fine-structure constant which governs the rate with which the muon spin precesses relative to the muon direction of motion in a static magnetic field. Measurements by the Muon g-2 collaboration at Brookhaven National Laboratory and at Fermilab revealed this small shift of roughly one part per million in the fine-structure constant using the spin precession of a beam of muons moving with nearly the speed of light around a large magnetic storage ring in April 2021.

Axion dark matter

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Axions formed in the Big Bang from the binding of normal antimatter to mirror matter.[1] For every photon made in the Big Bang, six axions formed. With a rest mass roughly one million times less than the electron, axions form the dark matter in the universe.

Axion Electrodynamics

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The axion couples to the scalar product of the electric field and the magnetic field with coupling strength [1]

 =0.676(18)×10−10 GeV-1.

This coupling of the axion to the electromagnetic field changes the equations of electrodynamics.[10] In "natural units", where the reduced Planck constant  , speed of light  , and permittivity of free space   all reduce to 1, the axion electrodynamic equations are:

Name Equations
Gauss's law  
Gauss's law for magnetism  
Faraday's law  
Ampère–Maxwell law  
Axion field's equation of motion  

Above, a dot above a variable denotes its time derivative and the dot spaced between variables is the vector dot product.

Condensed Matter Realizations of Axion Electrodynamics

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A term analogous to the one that would be added to Maxwell's equations to account for axions[11] also appears in recent (2008) theoretical models for topological insulators giving an effective axion description of the electrodynamics of these materials.[12]

This term leads to several interesting predicted properties including a quantized magnetoelectric effect.[13] Evidence for this effect has been given in THz spectroscopy experiments performed at the Johns Hopkins University on quantum regime thin film topological insulators developed at Rutgers University.[14]

In 2019, a team at the Max Planck Institute for Chemical Physics of Solids published their detection of an axion electrodynamic response in the charge density wave phase of a Weyl semimetal material.[15] The phase of the charge density wave couples to the scalar product of the static external magnetic field and the static applied electric field driving electrical current to flow in the material. This axion electrodynamic coupling generates a winding of the charge density wave phase that can be detected as a positive differential magnetoconductance whose size grows as the square of the magnetic field strength. [16]

Experiments

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The axion was first observed in the Muon g-2 experiment following the release of the Run 1 results at Fermilab in April 2021. Axions from the local dark matter halo of the galaxy couple to the virtual photon that dresses the electromagnetic vertex of the muon, [1] and thjs coupling of the halo axions to the virtual photon shifts the effective value of the electromagnetic fine-structure constant which governs the rate with which the muon spin precesses relative to the muon direction of motion in a static magnetic field. Measurements by the Muon g-2 collaboration at Brookhaven National Laboratory and at Fermilab revealed this small shift of roughly one part per million in the fine-structure constant using the spin precession of a beam of muons moving with nearly the speed of light around a large magnetic storage ring.


 
Constraints on the axion's coupling to the photon
 
Constraints on the axion's dimensionless coupling to electrons

Direct conversion in a magnetic field

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Several experiments search for astrophysical axions by the Primakoff effect, which converts axions to photons and vice versa in electromagnetic fields.

The Axion Dark Matter Experiment (ADMX) at the University of Washington uses a strong magnetic field to detect the possible weak conversion of axions to microwaves.[17] ADMX searches the galactic dark matter halo[18] for axions resonant with a cold microwave cavity. ADMX has excluded optimistic axion models in the 1.9–3.53 μeV range.[19][20][21] From 2013 to 2018 a series of upgrades[22] were done and it is taking new data, including at 4.9–6.2 μeV. In December 2021 it excluded the 3.3–4.2 μeV range for the KSVZ model.[23][24]

Other experiments of this type include DMRadio,[25] HAYSTAC,[26] CULTASK,[27] and ORGAN.[28] HAYSTAC completed the first scanning run of a haloscope above 20 μeV in the late 2010s.[26]

Polarized light in a magnetic field

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The Italian PVLAS experiment searches for polarization changes of light propagating in a magnetic field. The concept was first put forward in 1986 by Luciano Maiani, Roberto Petronzio and Emilio Zavattini.[29] A rotation claim[30] in 2006 was excluded by an upgraded setup.[31] An optimized search began in 2014.

Light shining through walls

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Another technique is so called "light shining through walls",[32] where light passes through an intense magnetic field to convert photons into axions, which then pass through metal and are reconstituted as photons by another magnetic field on the other side of the barrier. Experiments by BFRS and a team led by Rizzo ruled out an axion cause.[33] GammeV saw no events, reported in a 2008 Physics Review Letter. ALPS I conducted similar runs,[34] setting new constraints in 2010; ALPS II is being built in 2022.[35] OSQAR found no signal, limiting coupling[36] and will continue.

Astrophysical axion searches

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Axion-like bosons could have a signature in astrophysical settings. In particular, several works have proposed axion-like particles as a solution to the apparent transparency of the Universe to TeV photons.[37][38] It has also been demonstrated that, in the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., magnetars), photons will convert much more efficiently. This would in turn give rise to distinct absorption-like features in the spectra detectable by early 21st century telescopes.[39] A new (2009) promising means is looking for quasi-particle refraction in systems with strong magnetic gradients. In particular, the refraction will lead to beam splitting in the radio light curves of highly magnetized pulsars and allow much greater sensitivities than currently achievable.[40] The International Axion Observatory (IAXO) is a proposed fourth generation helioscope.[41]

Axions can resonantly convert into photons in the magnetospheres of neutron stars.[42] The emerging photons lie in the GHz frequency range and can be potentially picked up in radio detectors, leading to a sensitive probe of the axion parameter space. This strategy has been used to constrain the axion–photon coupling in the 5–11 μeV mass range, by re-analyzing existing data from the Green Bank Telescope and the Effelsberg 100 m Telescope.[43] A novel, alternative strategy consists in detecting the transient signal from the encounter between a neutron star and an axion minicluster in the Milky Way.[44]

Axions can be produced in the Sun's core when X-rays scatter in strong electric fields. The CAST solar telescope is underway, and has set limits on coupling to photons and electrons. Axions may be produced within neutron stars, by nucleon–nucleon bremsstrahlung. The subsequent decay of axions to gamma rays allows constraints on the axion mass to be placed from observations of neutron stars in gamma-rays using the Fermi LAT assuming that axions couple to nucleons with a particular form suggested by Dine et al. From an analysis of four neutron stars, Berenji et al. (2016) obtained a 95% confidence interval upper limit on the axion mass of 0.079 eV which rules out the suggested form of axion-nucleon coupling given the observed mass of the axion of 0.50580(2) eV. [1] [45] In 2021 it has been also suggested[46][47] that a reported[48] excess of hard X-ray emission from a system of neutron stars known as the magnificent seven could be explained as axion emission.

In 2016, a theoretical team from Massachusetts Institute of Technology devised a possible way of detecting axions using a strong magnetic field that need be no stronger than that produced in an MRI scanning machine. It would show variation, a slight wavering, that is linked to the mass of the axion. As of 2019, the experiment is being implemented by experimentalists at the university.[49]

In 2022 the polarized light measurements of Messier 87* by the EHT were used to constrain the mass of the axion assuming that hypothetical clouds of axions could form around a black hole, rejecting the approximate 10−21 eV/c2 – 10−20 eV/c2 range of mass values.[50][51]

Searches for resonance effects

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Resonance effects may be evident in Josephson junctions[52] from a supposed high flux of axions from the galactic halo with mass of 110 μeV and density 0.05 GeV/cm3[53] compared to the implied dark matter density 0.3±0.1 GeV/cm3, indicating said axions would not have enough mass to be the sole component of dark matter. The ORGAN experiment plans to conduct a direct test of this result via the haloscope method.[28]

Dark matter recoil searches

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Dark matter cryogenic detectors have searched for electron recoils that would indicate axions. CDMS published in 2009 and EDELWEISS set coupling and mass limits in 2013. UORE and XMASS also set limits on solar axions in 2013. XENON100 used a 225-day run to set the best coupling limits to date and exclude some parameters.[54]

Nuclear spin precession

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While Schiff's theorem states that a static nuclear electric dipole moment (EDM) does not produce atomic and molecular EDMs,[55] the axion induces an oscillating nuclear EDM that oscillates at the Larmor frequency. If this nuclear EDM oscillation frequency is in resonance with an external electric field, a precession in the nuclear spin rotation occurs. This precession can be measured using precession magnetometry and if detected, would be evidence for Axions.[56]

An experiment using this technique is the Cosmic Axion Spin Precession Experiment (CASPEr).[57][58][59]

Searches at particle colliders

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Axions may also be produced at colliders, in particular in electron positron collisions as well as in ultra-peripheral heavy ion collisions at the Large Hadron Collider at CERN, reinterpreting the light-by-light scattering process. Those searches are sensitive for rather large axion masses between 100 MeV/c2 and hundreds of GeV/c2. Assuming a coupling of axions to the Higgs Boson, searches for anomalous Higgs boson decays into two axions can theoretically provide even stronger limits.[60]

Disputed detections

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It was reported in 2014 that evidence for axions may have been detected as a seasonal variation in observed X-ray emission that would be expected from conversion in the Earth's magnetic field of axions streaming from the Sun. Studying 15 years of data by the European Space Agency's XMM-Newton observatory, a research group at Leicester University noticed a seasonal variation for which no conventional explanation could be found. One potential explanation for the variation, described as "plausible" by the senior author of the paper, is the known seasonal variation in visibility to XMM-Newton of the sunward magnetosphere in which X-rays may be produced by axions from the Sun's core.[61][62]

This interpretation of the seasonal variation is disputed by two Italian researchers, who identify flaws in the arguments of the Leicester group that are said to rule out an interpretation in terms of axions. Most importantly, the scattering in angle assumed by the Leicester group to be caused by magnetic field gradients during the photon production, necessary to allow the X-rays to enter the detector that cannot point directly at the sun, would dissipate the flux so much that the probability of detection would be negligible.[63]

In 2013, Christian Beck suggested that axions might be detectable in Josephson junctions; and in 2014, he argued that a signature, consistent with a mass ≈110 μeV, had in fact been observed in several preexisting experiments.[64]

In 2020, the XENON1T experiment at the Gran Sasso National Laboratory in Italy reported a result suggesting the discovery of solar axions.[65] The results are not yet significant at the 5-sigma level required for confirmation, and other explanations of the data are possible though less likely.[66] New observations made in July 2022, after the observatory upgrade to XENONnT, discarded the excess thus ending the possibility of new particle discovery.[67][68]

Properties

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Rest-Mass

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The axion has rest-mass 0.50580(2) eV/c2.[1]

Flavor

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Axions come in twelve flavors corresponding to the twelve flavors of quarks and leptons in the standard model of particle physics.[1]

Effect on fundamental constants

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Axions from the local dark matter halo of the galaxy couple to the virtual photon that dresses the electromagnetic vertex with matter.[1] This coupling between the virtual photon and the halo axions shifts the effective value of the electromagnetic fine-structure constant that sets the size of quantum electrodynamic effects. The analogous coupling of halo axions to the virtual gluon that dresses the electroweak vertex of quarks shifts the effective value of the strong coupling constant that sets the size of quantum chromodynamic effects.

Cosmological number density

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There are six axions for every photon in the cosmic microwave background.[1]

See also

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Footnotes

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  1. ^ This non-trivial vacuum structure solves a problem associated to the U(1) axial symmetry of QCD[3][4]
  2. ^ One simple solution to the strong CP problem exists: If at least one of the quarks of the standard model is massless, CP-violation becomes unobservable. However, empirical evidence strongly suggests that none of the quarks are massless. Consequently, particle theorists sought other resolutions to the problem of inexplicably conserved CP.

References

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