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{{Other uses}}
{{Infobox particle
| name = Axion
| composition =
| status = Hypothetical
| interaction = [[Gravity]], [[Electromagnetic force|electromagnetic]]
| theorized = 1977, [[Roberto Peccei|Peccei]] and [[Helen Quinn|Quinn]]
| symbol = A<sup>0</sup>
| mass = 10<sup>−5</sup> to 10<sup>−3</sup>&nbsp;[[electronvolt|eV]]/[[Speed of light|''c'']]<sup>2</sup>{{spaces|1}}<ref>{{Cite book|author=Peccei, R. D. |year=2008 |chapter=The Strong CP Problem and Axions |title=Axions: Theory, Cosmology, and Experimental Searches |journal=<!-- -->|editor1-last=Kuster |editor1-first=Markus |editor2-last=Raffelt |editor2-first=Georg |editor3-last=Beltrán |editor3-first=Berta |series=Lecture Notes in Physics |volume=741 |pages=3–17 |arxiv=hep-ph/0607268 |doi=10.1007/978-3-540-73518-2_1 |isbn=978-3-540-73517-5}}</ref>
| width = 10<sup>9</sup> to 10<sup>12</sup> [[GeV]]/c<sup>2</sup> <ref>{{cite journal|author=Duffy, Leanne D.|author2=van Bibber, Karl|title=Axions as dark matter particles|journal=New Journal of Physics|volume=11|issue=10|year=2009|page=105008|doi=10.1088/1367-2630/11/10/105008|arxiv=0904.3346|bibcode=2009NJPh...11j5008D}}</ref>
| electric_charge = 0
| spin = 0
| Grouping = [[Goldstone Boson]]
}}

The '''axion''' ({{IPAc-en|ˈ|æ|k|s|i|ɒ|n}}) 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]].

==History==
===Strong CP problem===
As shown by [[Gerard 't Hooft]],<ref>{{cite journal|author='t Hooft, G.|title=Symmetry breaking through Bell-Jackiw anomalies|journal=Phys. Rev. Lett.|volume=37|issue=1|year=1976}}{{cite journal|author='t Hooft, G.|title=Computation of the quantum effects due to a four-dimensional pseudo-particle|journal=Physical review: D|volume=14|issue=12|year=1976|pages=3432–3450|publisher=APS}}</ref> [[strong interaction]]s of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits violation of the combined symmetries of [[C-symmetry|charge conjugation]] and [[parity (physics)|parity]], collectively known as CP. Together with effects generated by [[weak interaction]]s, the effective periodic strong CP-violating term, {{overline|Θ}}, appears as a [[Standard Model]] input &ndash; its value is not predicted by the theory, but must be measured. However, large CP-violating interactions originating from QCD would induce a large [[Neutron electric dipole moment|electric dipole moment (EDM) for the neutron]]. Experimental constraints on the currently unobserved EDM implies CP violation from QCD must be extremely tiny and thus {{overline|Θ}} must itself be extremely small. Since a priori {{overline|Θ}} could have any value between 0 and 2''π'', 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]].

One simple solution 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.
<!-- The same paragraph appears in [[strong CP problem]]. Does it need to be repeated here? -->

===Prediction===
In 1977, [[Roberto Peccei]] and [[Helen Quinn]] postulated a more elegant solution to the strong CP problem, the [[Peccei–Quinn theory|Peccei–Quinn mechanism]]. The idea is to effectively promote {{overline|Θ}} to a field. This is accomplished by adding a new global symmetry (called a [[Peccei–Quinn symmetry]]) that becomes spontaneously broken. This results in a new particle, as shown by [[Frank Wilczek]] and [[Steven Weinberg]], that fills the role of {{overline|Θ}}, naturally relaxing the CP-violation parameter to zero. This hypothesized new particle is called the axion. The original Weinberg–Wilczek axion was ruled out.{{refn|name="pseudo-Nambu–Goldstone boson"|group=lower-alpha|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]].}}

=== Searches ===
Axion models carefully chose coupling that could not have been detected in prior experiments. It had been thought that these “''invisible axions''” solved the strong CP problem while still being too small to have been observed before. Current literature discusses “invisible axion” mechanisms in two forms, called KSVZ ([[Kim Jihn-eui|Kim]]–[[Mikhail Shifman|Shifman]]–[[Arkady Vainshtein|Vainshtein]]–{{nowrap|{{abbr|Zakharov|Valya Zakharov}})<ref>{{cite journal
 |last1=Kim |first1=J. E.
 |title=Weak-Interaction Singlet and Strong CP Invariance
 |year=1979
 |journal=Phys. Rev. Lett.
 |volume=43 |issue=2 |pages=103–107
 |bibcode=1979PhRvL..43..103K |doi=10.1103/PhysRevLett.43.103
}}</ref><ref>{{cite journal
 |last1=Shifman |first1=M.
 |last2=Vainshtein |first2=A.
 |last3=Zakharov |first3=V.
 |year=1980
 |journal=Nucl. Phys.
 |volume=B166 |issue=3
 |pages=493–506
 |title=Can confinement ensure natural CP invariance of strong interactions?
 |doi=10.1016/0550-3213(80)90209-6 |bibcode=1980NuPhB.166..493S
}}</ref>}} and DFSZ ([[Michael Dine|Dine]]–[[Willy Fischler|Fischler]]–{{abbr|Srednicki|Mark Srednicki}}–{{nowrap|{{abbr|Zhitnitsky|Ariel R. Zhitnitsky}}).<ref>{{cite journal
 |last1=Dine |first1=M.
 |last2=Fischler |first2=W.
 |last3=Srednicki |first3=M.
 |year=1981
 |journal=Phys. Lett.
 |volume=B104 |issue=3
 |pages=199–202
 |title=A simple solution to the strong CP problem with a harmless axion
 |doi=10.1016/0370-2693(81)90590-6  |bibcode=1981PhLB..104..199D
}}</ref><ref>{{cite journal
 |last1=Zhitnitsky |first1=A.
 |year=1980
 |journal=Sov. J. Nucl. Phys.
 |volume=31 |page=260
 |title=On possible suppression of the axion-hadron interactions
 |url=https://inis.iaea.org/Search/search.aspx?orig_q=RN:11563932
}}</ref>}}

The very weakly coupled axion is also very light because axion couplings and mass are proportional. Satisfaction with “invisible axions” changed when it was shown that any very light axion would have been overproduced in the early universe and therefore must be excluded. The critical mass is of order 10<sup>−11</sup> times the [[electron mass]].<ref>{{cite journal
 |last1=Preskill |first1=J. |authorlink1=John Preskill
 |last2=Wise |first2=M. |authorlink2=Mark B. Wise
 |last3=Wilczek |first3=F. |authorlink3=Frank Wilczek
 |date=6 January 1983
 |journal=Physics Letters B
 |volume=120 |issue=1–3 |pages=127–132
 |title=Cosmology of the invisible axion
 |doi=10.1016/0370-2693(83)90637-8
 |bibcode=1983PhLB..120..127P
 |url=http://www.theory.caltech.edu/~preskill/pubs/preskill-1983-axion.pdf
|citeseerx=10.1.1.147.8685 }}</ref><ref>{{cite journal
 |last1=Abbott |first1=L.
 |last2=Sikivie |first2=P.
 |year=1983
 |journal=Physics Letters B
 |volume=120 |issue=1–3 |pages=133–136
 |title=A cosmological bound on the invisible axion
 |bibcode=1983PhLB..120..133A
|doi = 10.1016/0370-2693(83)90638-X |citeseerx=10.1.1.362.5088
 }}</ref><ref>{{cite journal
 |last1=Dine |first1=M.
 |last2=Fischler |first2=W.
 |year=1983
 |journal=Physics Letters B
 |volume=120 |issue=1–3 |pages=137–141
 |title=The not-so-harmless axion
 |doi=10.1016/0370-2693(83)90639-1
 |bibcode=1983PhLB..120..137D
}}</ref>

With a mass above 10<sup>−11</sup> times the [[electron mass]], axions could account for [[dark matter]], thus be both a dark-matter candidate and a solution to the strong CP problem. 
A mass value between 0.05 and 1.50&nbsp;meV for the axion was reported in a paper published in November 2016 (Borsanyi, S. et al.).<ref>{{cite journal
 |last1=Borsanyi |first1=S.
 |display-authors=etal
 |year=2016
 |journal=Nature
 |volume=539 |issue=69–71 |pages=69–71
 |title=Calculation of the axion mass based on high-temperature lattice quantum chromodynamics
 |doi=10.1038/nature20115
|bibcode=2016Natur.539...69B
 |pmid=27808190
 |url=http://real.mtak.hu/61427/1/nature_axion.pdf
 }}</ref> The result was calculated by simulating the formation of axions during the [[Inflation (cosmology)|post-inflation]] period on a [[supercomputer]].<ref>{{Cite journal
 |url=http://www.nature.com/news/axion-alert-exotic-particle-detector-may-miss-out-on-dark-matter-1.20925#/b1
 |title=Axion alert! Exotic-particle detector may miss out on dark matter
 |first=Davide |last=Castelvecchi
 |date=3 November 2016
 |journal=[[Nature (journal)|Nature]]
|doi=10.1038/nature.2016.20925
 }}</ref>

===Maxwell's equations with axion modifications===
[[Pierre Sikivie]] published a modification of [[Maxwell's equations]] that arise from a light, stable axion in 1983.<ref>{{cite journal
 |last1=Sikivie |first1=P.
 |title=Experimental Tests of the 'Invisible' Axion
 |date=17 October 1983
 |journal=Phys. Rev. Lett.
 |volume=51 |issue=16 |page=1413
 |doi=10.1103/physrevlett.51.1415
 |bibcode=1983PhRvL..51.1415S
}}</ref> He showed that these axions could be detected on Earth by converting them to photons, using a strong magnetic field, hence leading to several experiments: the [[Axion Dark Matter Experiment|ADMX]]; Solar axions may be converted to X-rays, as in [[CERN Axion Solar Telescope|CAST]]; Other experiments are searching laser light for signs of axions.<ref>{{cite web |title=OSQAR |url=http://home.cern/about/experiments/osqar |publisher=CERN |date=2017 |accessdate=3 October 2017}}</ref>

If [[magnetic monopole]]s exist then there is a symmetry in Maxwell's equations where the electric and magnetic fields can be rotated into each other with the new fields still satisfying Maxwell's equations. Luca Visinelli showed that the duality symmetry can be carried over to the axion-electromagnetic theory as well. Assuming the existence of both magnetic charges and axions, Maxwell's equations read

{| class="wikitable" style="text-align: center;"
|-
! scope="col" style="width: 15em;" | Name
! scope="col" |  Equations
|-
! scope="row" | Gauss's law
| <math>\nabla \cdot (\mathbf{E} - c \kappa \theta \mathbf{B}) = \frac {\rho_e} {\varepsilon_0}</math>
|-
! scope="row" | Gauss's law for magnetism
| <math>\nabla \cdot (\mathbf{B} + \frac{\kappa}{c} \theta \mathbf{E}) = \mu_0 \rho_m </math>
|-
! scope="row" | Faraday's law
| <math>\nabla \times (\mathbf{E} - c\kappa \theta \mathbf{B}) = -\partial_t ( \mathbf{B} + \frac{\kappa}{c} \theta \mathbf{E} ) - \mu_0 \mathbf{J}_m </math>
|-
! scope="row" | Ampère–Maxwell law
| <math>\nabla \times ( \mathbf{B} + \frac{\kappa}{c} \theta \mathbf{E}) = \frac{1}{c^2} \partial_t ( \mathbf{E} - c \kappa \theta \mathbf{B} ) + \mu_0 \mathbf{J}_e </math>
|-
! scope="row" | Axion law
| <math> ( \Box + m_a^2) \theta = -\kappa \mathbf{E} \cdot \mathbf{B} </math>
|}

If [[magnetic monopole]]s do not exist, then the same equations hold with the density <math>\rho_m</math> and current <math>\mathbf{J}_m</math> replaced by zero. Incorporating the axion has the effect of rotating the electric and magnetic fields into each other.

{{block indent|<math> 
\begin{pmatrix}
\mathbf{E'} \\
\mathbf{B'} \\
\end{pmatrix}
 = \frac{1}{\cos \xi} 
\begin{pmatrix}
\cos \xi & c \sin \xi \\
-\frac{1}{c} \sin \xi & \cos \xi \\
\end{pmatrix}
\begin{pmatrix}
\mathbf{E} \\
\mathbf{B} \\
\end{pmatrix}
</math>}}

where the mixing angle <math>\xi</math> depends on the coupling constant <math>\kappa</math> and the axion field strength <math>\theta</math>

{{block indent|<math>
\tan \xi = -\kappa \theta
</math>}}

By plugging the new values for electromagnetic field <math>\mathbf{E'}</math> and <math>\mathbf{B'}</math> into Maxwell's equations we obtain the axion-modified Maxwell equations above.  Incorporating the axion into the electromagnetic theory also gives a new differential equation—the axion law—which is simply the [[Klein–Gordon equation]] (the quantum field theory equation for massive spin-zero particles) with an <math>\mathbf{E} \cdot \mathbf{B}</math> source term.

A term analogous to the one that would be added to [[Maxwell's equations]] to account for axions<ref>{{Cite journal
| volume = 58
| issue = 18
| pages = 1799–1802
| last = Wilczek
| first = Frank
| title = Two applications of axion electrodynamics
| journal = Physical Review Letters
| date = 4 May 1987
| doi = 10.1103/PhysRevLett.58.1799
| pmid = 10034541
| bibcode=1987PhRvL..58.1799W
}}</ref> also appears in recent (2008) theoretical models for [[topological insulators]] giving an effective axion description of the electrodynamics of these materials.<ref>{{Cite journal
| journal = Physical Review B
| volume = 78
| issue = 19
| page = 195424
| last1 = Qi | first1 = Xiao-Liang
| last2 = Hughes | first2 = Taylor L.
| last3 = Zhang | first3 = Shou-Cheng
| title = Topological field theory of time-reversal invariant insulators
| date = 24 November 2008
| doi = 10.1103/PhysRevB.78.195424
| bibcode = 2008PhRvB..78s5424Q |arxiv = 0802.3537
}}</ref>  This term leads to several interesting predicted properties including a quantized [[Magnetoelectric effect|magnetoelectric]] effect.<ref name="Franz 36">{{Cite journal
| doi = 10.1103/Physics.1.36
| volume = 1
| page = 36
| last = Franz | first = Marcel
| title = High-energy physics in a new guise
| journal = Physics
| date = 24 November 2008
| bibcode = 2008PhyOJ...1...36F
}}</ref> Evidence for this effect has recently been given in [[N. Peter Armitage|THz spectroscopy experiments]] performed at the [[Johns Hopkins University]].<ref>{{Cite journal|last=Wu|first=Liang|last2=Salehi|first2=M.|last3=Koirala|first3=N.|last4=Moon|first4=J.|last5=Oh|first5=S.|last6=Armitage|first6=N. P.|date=2 December 2016|title=Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator|journal=Science|language=en|volume=354|issue=6316|pages=1124–1127|doi=10.1126/science.aaf5541|issn=0036-8075|pmid=27934759|arxiv=1603.04317|bibcode=2016Sci...354.1124W}}</ref>

== Experiments ==
The Italian [[PVLAS]] experiment searches for polarization changes of [[polarized light|light]] propagating in a magnetic field. The concept was first put forward in 1986 by [[Luciano Maiani]], Roberto Petronzio and [[Emilio Zavattini]].<ref>{{cite journal
 |last1=Maiani |first1=L. |authorlink1=Luciano Maiani
 |last2=Petronzio |first2=R.
 |last3=Zavattini |first3=E. |authorlink3=Emilio Zavattini
 |date=7 August 1986
 |title=Effects of nearly massless, spin-zero particles on light propagation in a magnetic field
 |url=http://cdsweb.cern.ch/record/167556/files/198606014.pdf
 |journal=Physics Letters B
 |volume=175 |issue=3 |pages=359–363
 |doi=10.1016/0370-2693(86)90869-5
 |bibcode=1986PhLB..175..359M
 |id=CERN-TH.4411/86}}</ref> A rotation claim<ref>{{cite magazine |first1=Steve |last1=Reucroft |first2=John |last2=Swain |url=http://cerncourier.com/main/article/46/8/10 |title=Axion signature may be QED |archive-url=https://web.archive.org/web/20080820154706/http://cerncourier.com/main/article/46/8/10 |archive-date=2008-08-20 |magazine=CERN Courier |date=2006-10-05 |df=dmy-all}}</ref> in 2006 was excluded by an upgraded setup.<ref name="zavattini_1">{{cite journal
 |doi=10.1103/PhysRevLett.96.110406
 |pmid=16605804
 |title=Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field
 |year=2006
 |last1=Zavattini |first1=E.
 |collaboration=PVLAS Collaboration
 |journal=Physical Review Letters
 |volume=96 |issue=11 |page=110406
 |url=http://cecelia.physics.indiana.edu/journal/pvlas.pdf
 |arxiv=hep-ex/0507107 |bibcode=2006PhRvL..96k0406Z
}}</ref> An optimized search began in 2014.

Another technique is so called "light shining through walls",<ref>{{cite conference
 |doi=10.1142/9789812704214_0007
 |date=16–21 October 2001
 |last1=Ringwald |first1=A.
 |title=Fundamental Physics at an X-Ray Free Electron Laser
 |conference=Workshop on Electromagnetic Probes of Fundamental Physics
 |location= [[Erice]], Italy
 |arxiv=hep-ph/0112254 
 |book-title=Electromagnetic Probes of Fundamental Physics – Proceedings of the Workshop
 |pages=63–74
 |isbn=978-981-238-566-6
}}</ref> where light passes through an intense magnetic field to convert photons into axions, that pass through metal. Experiments by BFRS and a team led by Rizzo ruled out an axion cause.<ref name="rizzo_1">{{cite journal
 |arxiv=0707.1296
 |doi=10.1103/PhysRevLett.99.190403
 |pmid=18233050
 |title=No 'Light Shining through a Wall': Results from a Photoregeneration Experiment
 |year=2007
 |last1=Robilliard |first1=C.
 |last2=Battesti |first2=R.
 |last3=Fouche |first3=M.
 |last4=Mauchain |first4=J.
 |last5=Sautivet |first5=A.-M.
 |last6=Amiranoff |first6=F.
 |last7=Rizzo |first7=C.
 |journal=Physical Review Letters
 |volume=99 |issue=19| page=190403
 |bibcode=2007PhRvL..99s0403R
}}</ref> GammeV saw no events in a 2008 PRL. ALPS-I conducted similar runs,<ref>{{Cite journal
 |url=http://pubman.mpdl.mpg.de/pubman/faces/viewItemOverviewPage.jsp?itemId=escidoc:417635
 |title=New ALPS results on hidden-sector lightweights
 |date=May 2010
 |journal=Phys. Lett. B
 |volume=689 |issue=4–5 |pages=149–155
 |doi=10.1016/j.physletb.2010.04.066
 |last1=Ehret |first1=Klaus
 |last2=Frede |first2=Maik
 |last3=Ghazaryan |first3=Samvel
 |last4=Hildebrandt |first4=Matthias
 |last5=Knabbe |first5=Ernst-Axel
 |last6=Kracht |first6=Dietmar
 |last7=Lindner |first7=Axel
 |last8=List |first8=Jenny
 |last9=Meier |first9=Tobias
 |last10=Meyer |first10=Niels
 |last11=Notz |first11=Dieter
 |last12=Redondo |first12=Javier
 |last13=Ringwald |first13=Andreas
 |last14=Wiedemann |first14=Günter
 |last15=Willke |first15=Benno
 |bibcode=2010PhLB..689..149E |arxiv=1004.1313}}</ref> setting new constraints in 2010; ALPS-II will run in 2019.{{Update after|2018}} OSQAR found no signal, limiting coupling<ref>{{Cite journal
 |title=Search for weakly interacting sub-eV particles with the OSQAR laser-based experiment: results and perspectives
 |date=Aug 2014 
 |journal=Eur Phys J C
 |volume=74 |issue=8 |pages=3027
 |doi=10.1140/epjc/s10052-014-3027-8
 |last1=Pugnat |first1=P.
 |last2=Ballou |first2=R.
 |last3=Schott |first3=M. 
 |last4=Husek |first4=T.
 |last5=Sulc |first5=M.
 |last6=Deferne |first6=G.
 |last7=Duvillaret |first7=L.
 |last8=Finger |first8=M.
 |last9=Finger|first9=M.
 |last10=Flekova |first10=L.
 |last11=Hosek |first11=J.
 |last12=Jary |first12=V.
 |last13=Jost |first13=R.
 |last14=Kral |first14=M.
 |last15=Kunc |first15=S.
 |last16=MacUchova |first16=K.
 |last17=Meissner |first17=K. A.
 |last18=Morville |first18=J.
 |last19=Romanini |first19=D.
 |last20=Siemko |first20=A.
 |last21=Slunecka |first21=M.
 |last22=Vitrant |first22=G.
 |last23=Zicha  |first23=J.
 |bibcode=2014EPJC...74.3027P
 |arxiv=1306.0443}}</ref> and will continue.

Several experiments search for astrophysical axions by the [[Primakoff effect]], which converts axions to photons and vice versa in electromagnetic fields. Axions can be produced in the Sun's core when X-rays scatter in strong electric fields. The [[CERN Axion Solar Telescope|CAST]] solar telescope is underway, and has set limits on coupling to photons and electrons. [[Axion Dark Matter Experiment|ADMX]] searches the galactic [[dark matter halo]]<ref>{{cite journal |arxiv=astro-ph/0603108 |doi=10.1103/PhysRevD.74.012006 |title=High resolution search for dark-matter axions |year=2006 |last1=Duffy |first1=L. D. |last2=Sikivie |first2=P. |last3=Tanner |first3=D. B. |last4=Bradley |first4=R. F. |last5=Hagmann |first5=C. |last6=Kinion |first6=D. |last7=Rosenberg |first7=L. J. |last8=Van Bibber |first8=K. |last9=Yu |first9=D. B. |last10=Bradley |first10=R. F. |journal=Physical Review D|volume=74 |issue=1 |page=12006 |bibcode=2006PhRvD..74a2006D }}</ref> for resonant axions with a cold microwave cavity and has excluded optimistic axion models in the 1.9–3.53&nbsp;μeV range.<ref>{{cite journal |arxiv=0910.5914 |doi=10.1103/PhysRevLett.104.041301 |title=SQUID-Based Microwave Cavity Search for Dark-Matter Axions |year=2010 |last1=Asztalos |first1 = S. J. |last2=Carosi |first2=G. |last3=Hagmann |first3=C. |last4=Kinion |first4=D. |last5=Van Bibber |first5=K. |last6=Hoskins |first6=J. |last7=Hwang |first7=J. |last8=Sikivie |first8=P. |last9=Tanner |first9=D. B. |last10=Hwang |first10=J. |last11=Sikivie |first11=P. |last12=Tanner |first12=D. B. |last13=Bradley |first13=R. |last14=Clarke |first14=J. |journal=Physical Review Letters |volume=104 |issue=4 |page=41301 |bibcode=2010PhRvL.104d1301A |pmid=20366699 |url=https://digital.library.unt.edu/ark:/67531/metadc1012348/m2/1/high_res_d/986065.pdf}}</ref><ref>{{cite web |url=http://www.phys.washington.edu/groups/admx/home.html |title=ADMX &#124; Axion Dark Matter eXperiment |publisher=Phys.washington.edu |accessdate=2014-05-10 |df=dmy-all}}</ref><ref>{{cite web |url=http://www.phys.washington.edu/groups/admx/results.html |title=Phase 1 Results |date=2006-03-04 |df=dmy-all}}</ref> It is amidst a series of upgrades and is taking new data, including at 4.9–6.2&nbsp;µeV. Other experiments of this type include HAYSTAC,<ref name=HAYSTAC>{{cite journal | last=Brubaker | first=B. M. | last2=Zhong | first2=L. | last3=Gurevich | first3=Y. V. | last4=Cahn | first4=S. B. | last5=Lamoreaux | first5=S. K. | last6=Simanovskaia | first6=M. | last7=Root | first7=J. R. | last8=Lewis | first8=S. M. | last9=Al Kenany | first9=S. | last10=Backes | first10=K. M. | last11=Urdinaran | first11=I. | last12=Rapidis | first12=N. M. | last13=Shokair | first13=T. M. | last14=van Bibber | first14=K. A. | last15=Palken | first15=D. A. | last16=Malnou | first16=M. | last17=Kindel | first17=W. F. | last18=Anil | first18=M. A. | last19=Lehnert | first19=K. W. | last20=Carosi | first20=G. | title=First Results from a Microwave Cavity Axion Search at 24&nbsp;μeV | journal=Physical Review Letters | volume=118 | issue=6 | pages=061302 | date=2017-02-09 | issn=0031-9007 | doi=10.1103/physrevlett.118.061302 | pmid=28234529 | ref=harv|arxiv = 1610.02580 |bibcode = 2017PhRvL.118f1302B |df=dmy-all}}</ref> CULTASK,<ref name=CULTASK>{{cite journal | last=Petrakou | first=Eleni | title=Haloscope searches for dark matter axions at the Center for Axion and Precision Physics Research | journal=EPJ Web of Conferences | volume=164 | pages=01012 | date=2017-02-13 | url=https://inspirehep.net/record/1513138 | ref=harv | access-date=2017-08-04| doi=10.1051/epjconf/201716401012 | bibcode=2017EPJWC.16401012P | arxiv=1702.03664 |df=dmy-all}}</ref> and ORGAN.<ref name=ORGAN>{{Cite journal| last=McAllister | first=Ben T. | last2=Flower | first2=Graeme | last3=Kruger | first3=Justin | last4=Ivanov | first4=Eugene N. | last5=Goryachev | first5=Maxim | last6=Bourhill | first6=Jeremy | last7=Tobar | first7=Michael E. | title=The ORGAN Experiment: An axion haloscope above 15&nbsp;GHz | journal=Physics of the Dark Universe | volume=18 | pages=67–72 | date=2017-06-01 | ref=harv | access-date=| arxiv=1706.00209 | bibcode=2017PDU....18...67M | doi=10.1016/j.dark.2017.09.010 }}</ref> HAYSTAC recently completed the first scanning run of a haloscope above 20&nbsp;µeV.<ref name=HAYSTAC />

Resonance effects may be evident in [[Josephson junction]]s<ref>{{cite journal |last=Beck |first=Christian |title=Possible Resonance Effect of Axionic Dark Matter in Josephson Junctions |journal=Physical Review Letters |date=December 2, 2013 |volume=111 |issue=23 |page=1801 |doi=10.1103/PhysRevLett.111.231801 |arxiv=1309.3790 |bibcode=2013PhRvL.111w1801B |pmid=24476255 |df=dmy-all}}</ref> from a supposed high flux of axions from the galactic halo with mass of 0.11&nbsp;meV and density {{Val|0.05|u=GeV⋅cm<sup>−3</sup>}}<ref>{{cite web |last=Moskvitch |first=Katia|title=Hints of cold dark matter pop up in 10&nbsp;year-old circuit |url=https://www.newscientist.com/article/dn24689-hints-of-cold-dark-matter-pop-up-in-10yearold-circuit.html |publisher=New Scientist magazine (Reed Business Information) |accessdate=3 December 2013}}</ref> compared to the implied dark matter density {{Val|0.3|0.1|u=GeV⋅cm<sup>−3</sup>}}, 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.<ref name=ORGAN />

Dark matter cryogenic detectors have searched for electron recoils that would indicate axions. [[Cryogenic Dark Matter Search|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&nbsp;day run to set the best coupling limits to date and exclude some parameters.<ref>{{Cite journal
 |url=https://science.purdue.edu/xenon1t/?p=292
 |title=First axion results from the XENON100 experiment
 |date=9 September 2014
 |journal=Phys. Rev. D
 |volume=90
 |issue=6
 |pages=062009
 |doi=10.1103/PhysRevD.90.062009
 |bibcode=2014PhRvD..90f2009A
 |arxiv=1404.1455
 |first1=E. |last1=Aprile
 |display-authors=etal
}}</ref>

Axion-like bosons could have a signature in astrophysical settings. In particular, several recent works have proposed axion-like particles as a solution to the apparent transparency of the Universe to TeV photons.<ref>{{cite journal |doi=10.1103/PhysRevD.76.121301 |arxiv=0707.4312 |title=Evidence for a new light spin-zero boson from cosmological gamma-ray propagation? |year=2007 |last1=De Angelis |first1=A. |last2=Mansutti |first2=O. |last3=Roncadelli |first3=M. |journal=Physical Review D |volume=76 |issue=12 |page=121301 |bibcode=2007PhRvD..76l1301D}}</ref><ref>{{cite journal |doi=10.1111/j.1745-3933.2008.00602.x |arxiv=0807.4246 |title=Photon propagation and the very high energy gamma-ray spectra of blazars: How transparent is the Universe? |year=2009 |last1=De Angelis |first1=A. |last2=Mansutti |first2=O. |last3=Persic |first3=M. |last4=Roncadelli |first4=M. |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=394 |issue=1 |pages=L21–L25 |bibcode=2009MNRAS.394L..21D}}
</ref> It has also been demonstrated in a few recent works that, in the large magnetic fields threading the atmospheres of compact astrophysical objects (e.g., [[magnetar]]s), photons will convert much more efficiently. This would in turn give rise to distinct absorption-like features in the spectra detectable by current telescopes.<ref>{{cite journal |doi=10.1088/0067-0049/180/1/1 |title=Spectral Signatures of Photon-Particle Oscillations from Celestial Objects |year=2009 |last1=Chelouche |first1=Doron |last2=Rabadan |first2=Raul |last3=Pavlov |first3=Sergey S. |last4 =Castejon |first4=Francisco |journal=The Astrophysical Journal Supplement Series |volume=180 |issue=1 |pages=1–29 |arxiv=0806.0411 |bibcode=2009ApJS..180....1C}}</ref> A new 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.<ref>{{cite journal |doi=10.1088/0004-637X/699/1/L5 |arxiv=0810.3002 |title=Cosmic analogs of the Stern-Gerlach experiment and the detection of light bosons |year=2009 |last1=Chelouche |first1=Doron |last2=Guendelman |first2=Eduardo I. |journal=The Astrophysical Journal |volume=699 |issue=1 |pages=L5–L8 |bibcode=2009ApJ...699L...5C}}</ref> The [[International Axion Observatory]] (IAXO) is a proposed fourth generation [[helioscope]].<ref>{{cite web |publisher=CERN |title=The International Axion Observatory |url=http://iaxo.web.cern.ch/content/home-international-axion-observatory |access-date=March 19, 2016 |df=dmy-all}}</ref>

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. From an analysis of four neutron stars, Berenji et al. obtained a 95% [[confidence interval]] upper limit on the axion mass of 0.079&nbsp;eV.<ref>{{cite journal |last2=Gaskins |first2=J. |last3=Meyer |first3=M. |year=2016 |title=Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars |journal=Physical Review D |volume=93 |issue=14 |page=045019 |arxiv=1602.00091 |doi=10.1103/PhysRevD.93.045019 |first1=B. |last1=Berenji |bibcode=2016PhRvD..93d5019B}}</ref>

==Possible detection==

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.<ref name=guardian>{{cite news|last1=Sample|first1=Ian|title=Dark matter may have been detected – streaming from sun's core|url=https://www.theguardian.com/science/2014/oct/16/dark-matter-detected-sun-axions|newspaper=The Guardian|accessdate=16 October 2014|date=2014-10-16}}</ref><ref name="FraserRead2014">{{cite journal|last1=Fraser|first1=G. W.|last2=Read|first2=A. M.|last3=Sembay|first3=S.|last4=Carter|first4=J. A.|last5=Schyns|first5=E.|title=Potential solar axion signatures in X-ray observations with the XMM-Newton observatory|journal=Monthly Notices of the Royal Astronomical Society|volume=445|issue=2|year=2014|pages=2146–2168|issn=0035-8711|doi=10.1093/mnras/stu1865|bibcode = 2014MNRAS.445.2146F |arxiv=1403.2436}}</ref> 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.<ref name="RoncadelliTavecchio2015">{{cite journal|last1=Roncadelli|first1=M.|last2=Tavecchio|first2=F.|title=No axions from the Sun|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=450|issue=1|year=2015|pages=L26–L28|issn=1745-3925|doi=10.1093/mnrasl/slv040|arxiv = 1411.3297 |bibcode = 2015MNRAS.450L..26R }}</ref>

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&nbsp;μeV, had in fact been observed in several preexisting experiments.<ref>{{Cite journal |doi = 10.1016/j.dark.2015.03.002 |arxiv = 1403.5676 |title=Axion mass estimates from resonant Josephson junctions |first=Christian |last=Beck |journal=Physics of the Dark Universe |volume = 7–8 |pages = 6–11 |date=2015 |bibcode = 2015PDU.....7....6B }}</ref>

In 2016 a theoretical team from [[Massachusetts Institute of Technology|MIT]] devised a possible way of detecting axions using a strong magnetic field. The magnetic field need be no stronger than that produced in a [[Magnetic resonance imaging|MRI]] scanning machine and it should show a slight wavering variation that is linked to the mass of the axion. The experiment is now being implemented by experimentalists at the university. Another approach being used by the [[University of Washington]] uses a strong magnetic field to detect the possible weak conversion of axions to [[microwave]]s.<ref>{{Cite news|url=http://phys.org/news/2016-10-team-simulates-magnetar-dark-particle.html#nRlv|title=Team simulates a magnetar to seek dark matter particle|access-date=2016-10-09}}</ref>

==Properties==

=== Predictions ===
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<sup>−6</sup> to {{val|1|u=eV/c2}}, and very low interaction [[cross section (physics)|cross-sections]] for [[strong interaction|strong]] and [[weak interaction|weak]] forces. Because of their properties, axions would interact only minimally with ordinary matter. Axions would also change to and from [[photon]]s in magnetic fields.

=== Supersymmetry ===
In [[supersymmetry|supersymmetric theories]] the axion has both a scalar and a fermionic [[superpartner]]. The [[fermion]]ic superpartner of the axion is called the [[axino]], the scalar superpartner is called the [[saxion]] or [[dilaton]].
They are all bundled up in a [[chiral superfield]].

The [[axino]] has been predicted to be the [[lightest supersymmetric particle]] in such a model.<ref>
{{cite journal
 |author=Abe, Nobutaka
 |author2=Takeo Moroi
 |author3=Masahiro Yamaguchi
 |last-author-amp=yes
 |year=2002
 |title=Anomaly-Mediated Supersymmetry Breaking with Axion
 |journal=[[Journal of High Energy Physics]]
 |volume=1 |issue=1
 |page=10
 |doi=10.1088/1126-6708/2002/01/010
|arxiv = hep-ph/0111155 |bibcode = 2002JHEP...01..010A 
 }}</ref>  In part due to this property, it is considered a candidate for [[dark matter]].<ref>
{{cite journal
 |author=Hooper, Dan |author2=Lian-Tao Wang
 |year=2004
 |title=Possible evidence for axino dark matter in the galactic bulge
 |journal=[[Physical Review D]]
 |volume=70 |page=063506
 |doi=10.1103/PhysRevD.70.063506
 |issue=6
|arxiv = hep-ph/0402220 |bibcode = 2004PhRvD..70f3506H 
 }}</ref>

=== Cosmological implications ===
Inflation suggests that axions were created abundantly during the [[Big Bang]].<ref>{{Cite journal |last1=Redondo |first1=J. |last2=Raffelt |first2=G. |last3=Viaux Maira |first3=N. |year=2012 |title=Journey at the axion meV mass frontier |url=http://iopscience.iop.org/1742-6596/375/2/022004 |journal=[[Journal of Physics: Conference Series]] |volume=375 |issue=2 |doi=10.1088/1742-6596/375/1/022004 |page=022004 |bibcode=2012JPhCS.375b2004R}}</ref> 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 (since there's no lighter particles to decay into), theories{{which|date=May 2016}} predict that the universe would be filled with a very cold [[Bose–Einstein condensate]] of primordial axions. Hence, axions could plausibly explain the [[dark matter]] problem of [[physical cosmology]].<ref>{{Cite journal |arxiv=0909.0949 |last1=Sikivie |first1=P. |title=Dark matter axions |journal=International Journal of Modern Physics A |volume=25 |issue=2n03 |pages=554–563 |year=2009 |doi=10.1142/S0217751X10048846 |bibcode=2010IJMPA..25..554S}}</ref> Observational studies are underway, but they are not yet sufficiently sensitive to probe the mass regions if they are the solution to the dark matter problem. High mass axions of the kind searched for by Jain and Singh (2007)<ref>{{cite journal |first1=P. L. |last1=Jain |first2=G. |last2=Singh |title=Search for new particles decaying into electron pairs of mass below 100&nbsp;''MeV''/c<sup>2</sup> |journal=J. Phys. G |series=Nucl. Part. Phys. |volume=34 |pages=129–138 |year=2007 |doi=10.1088/0954-3899/34/1/009 |quote=possible early evidence of 7±1 and 19±1&nbsp;MeV axions of less than 10<sup>−13</sup>&nbsp;s lifetime}}</ref> would not persist in the modern universe. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.<ref>{{cite journal |last=Salvio |title=Thermal axion production |journal=Journal of Cosmology and Astroparticle Physics |volume=2014 |issue=1 |pages=11 |url=http://inspirehep.net/record/1262100 |arxiv=1310.6982 |bibcode=2014JCAP...01..011S |doi=10.1088/1475-7516/2014/01/011 |year=2014 |first1=Alberto |last2=Strumia |first2=Alessandro |last3=Xue |first3=Wei}}</ref>

Low mass axions could have additional structure at the galactic scale. If they continuously fall into galaxies from the intergalactic medium, they would be denser in "[[Caustic (mathematics)|caustic]]" rings, just as the stream of water in a continuously-flowing fountain is thicker at its peak.<ref>{{cite web |url=http://www.osti.gov/bridge/purl.cover.jsp?purl=/548855-tz8W8A/webviewable/ |first=P. |last=Sikivie |title=Dark matter axions and caustic rings}}</ref> The gravitational effects of these rings on galactic structure and rotation might then be observable.<ref>{{cite web |author=P. Sikivie (personal website) |url=http://www.phys.ufl.edu/~sikivie/triangle/ |title=pictures of alleged triangular structure in Milky Way}}</ref><ref>{{cite web |url=http://www.phys.ufl.edu/~tanner/Axions2010/Duffy_Axions2010.pdf |title=Duffy (2010) "Axions" |quote=hypothetical flow diagram which could give rise to such a structure |url-status=dead}}</ref> Other cold dark matter theoretical candidates, such as [[Weakly interacting massive particles|WIMP]]s and [[MACHO]]s, could also form such rings, but because such candidates are [[fermion]]ic and thus experience friction or scattering among themselves, the rings would be less pronounced.

Ultralight axion (ULA) with  ''m''&nbsp;~&nbsp;10<sup>&minus;22</sup>&nbsp;eV  is a kind of [[scalar field dark matter]] which seems to solve the small scale problems of CDM. A single ULA with a GUT scale decay constant provides the correct relic density without fine-tuning.<ref>{{Cite journal |last=Marsh |first=David J.E. |date=2016 |title=Axion cosmology |journal=Physics Reports |language=en |volume=643 |pages=1–79 |doi=10.1016/j.physrep.2016.06.005|arxiv=1510.07633 }}</ref>

Axions would also have stopped interaction with normal matter at a different moment than other more massive dark particles.{{why|date=May 2016}} The lingering effects of this difference could perhaps be calculated and observed astronomically.

João G. Rosa and Thomas W. Kephart suggested that axion clouds formed around unstable [[primordial black hole]]s might initiate a chain of reactions that radiate electromagnetic waves, allowing their detection. When adjusting the mass of the axions to explain [[dark matter]], the pair discovered that the value would also explain the luminosity and wavelength of [[fast radio burst]]s, being a possible origin for both phenomena.<ref>{{cite journal |last1=Rosa |first1=João G. |last2=Kephart |first2=Thomas W. |year=2018 |title=Stimulated axion decay in superradiant clouds around primordial black holes |journal=Physical Review Letters |volume=120 |issue=23 |pages=231102 |arxiv=1709.06581 |doi=10.1103/PhysRevLett.120.231102 |pmid=29932720 |bibcode=2018PhRvL.120w1102R}}</ref>

==References==
===Footnotes===
{{notelist}}

===Citations===
{{Reflist|colwidth=30em}}

===Journal entries===
*{{cite journal |year=1977 |title=''CP'' conservation in the presence of pseudoparticles |journal=Physical Review Letters |volume=38 |issue=25 |pages=1440–1443 |doi=10.1103/PhysRevLett.38.1440 |url= |accessdate= |bibcode=1977PhRvL..38.1440P |last1=Peccei |first1=R.D. |authorlink1=Roberto Peccei |last2 = Quinn |first2=H.R. |authorlink2=Helen Quinn}}
*{{cite journal |author=Peccei, R. D. |author2=Quinn, H. R. |year=1977 |title=Constraints imposed by ''CP'' conservation in the presence of pseudoparticles |journal=Physical Review |volume=D16 |issue=6 |pages=1791–1797 |doi=10.1103/PhysRevD.16.1791 |url= |accessdate= |bibcode = 1977PhRvD..16.1791P }}
*{{cite journal |last=Weinberg |first=Steven |authorlink=Steven Weinberg |year=1978 |title=A new light boson? |journal=Physical Review Letters |volume=40 |issue=4 |pages=223–226 |doi=10.1103/PhysRevLett.40.223 |bibcode=1978PhRvL..40..223W}}
*{{cite journal |last=Wilczek |first=Frank |authorlink=Frank Wilczek |year=1978 |title=Problem of strong ''P'' and ''T'' invariance in the presence of instantons |journal=Physical Review Letters |volume=40 |issue=5 |pages=279–282 |doi=10.1103/PhysRevLett.40.279 |bibcode=1978PhRvL..40..279W}}

==External links==
*[http://physics.aps.org/articles/v1/36 November 24, 2008 article in APS Physics]
*[https://www.newscientist.com/channel/fundamentals/dn11041-lastgasp-test-could-reveal-dark-matter.html January 28, 2007 news article by newscientist.com]
*[https://web.archive.org/web/20061207214335/http://www.physorg.com/news84633896.html December 06, 2006 news article by physorg.com]
*[http://sciam.com/article.cfm?chanID=sa004&articleID=0006BA85-FC58-1492-BAAC83414B7F0000&ref=nature July 17, 2006 news article from Scientific American]
*[http://physicsweb.org/articles/news/10/3/19/1 March 27, 2006 news article by PhysicsWeb.org]
*[http://physicsweb.org/articles/news/8/11/13/1 November 24, 2004 news article by PhysicsWeb.org]
*[http://cern.ch/cast CAST Experiment]
* [http://gifna.unizar.es/cast/ CAST at UNIZAR]
*[https://web.archive.org/web/20090318035648/http://astropp.physik.tu-darmstadt.de/cast/ CAST at University of Technology Darmstadt]
*[http://www.phys.washington.edu/groups/admx/home.html ADMX at University of Washington]
*[https://ncatlab.org/nlab/show/axion Axion in nLab]

{{Particles}}
{{Dark matter}}

[[Category:Bosons]]
[[Category:Dark matter]]
[[Category:Astroparticle physics]]
[[Category:Hypothetical elementary particles]]