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== Experiments == Despite not having been found to date, the axion has been well studied for over 40 years, giving time for physicists to develop insight into axion effects that might be detected. Several experimental searches for axions are presently underway; most exploit axions' expected slight interaction with photons in strong magnetic fields. Axions are also one of the few remaining plausible candidates for dark matter particles, and might be discovered in some dark matter experiments. [[File:AxionPhoton.pdf|thumb|upright=1.6|Constraints on the axion's coupling to the photon]] [[File:AxionElectron.pdf|thumb|upright=1.6|Constraints on the axion's dimensionless coupling to electrons]] === Direct conversion in a magnetic field === 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]] is a [[haloscope]] that uses a strong magnetic field to detect the possible weak conversion of axions to [[microwave]]s.<ref>{{cite press release |last1=Chu |first1=Jennifer |title=Team simulates a magnetar to seek dark matter particle |url=https://phys.org/news/2016-10-team-simulates-magnetar-dark-particle.html |work=Phys.org |publisher=Massachusetts Institute of Technology }}</ref> ADMX searches the galactic [[dark matter halo]]<ref> {{cite journal |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. |display-authors=6 |year=2006 |title=High resolution search for dark-matter axions |journal=Physical Review D |volume=74 |issue=1 |page=12006 |arxiv=astro-ph/0603108 |doi=10.1103/PhysRevD.74.012006 |bibcode=2006PhRvD..74a2006D |s2cid=35236485 }} </ref> for axions resonant with a cold microwave cavity. ADMX has excluded optimistic axion models in the range {{val|1.9|–|3.53|u=μeV}}.<ref> {{cite journal |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. |display-authors=6 |year=2010 |title=SQUID-based microwave cavity search for dark-matter axions |url=https://digital.library.unt.edu/ark:/67531/metadc1012348/m2/1/high_res_d/986065.pdf |journal=Physical Review Letters |volume=104 |issue=4 |page=41301 |arxiv=0910.5914 |bibcode=2010PhRvL.104d1301A |doi=10.1103/PhysRevLett.104.041301 |pmid=20366699 |s2cid=35365606}} </ref><ref> {{cite web |title=ADMX {{pipe}} Axion Dark Matter eXperiment |url=http://www.phys.washington.edu/groups/admx/home.html |access-date=2014-05-10 |website=phys.washington.edu |publisher=University of Washington |place=Seattle, Washington |department=Physics |df=dmy-all}} </ref><ref> {{cite web |date=2006-03-04 |title=Phase 1 results |url=http://www.phys.washington.edu/groups/admx/results.html |website=phys.washington.edu |publisher=University of Washington |place=Seattle, Washington |df=dmy-all |department=Physics}}</ref> From 2013 to 2018 a series of upgrades<ref>{{cite tech report | doi=10.2172/1508642 | title=The "Gen 2" Axion Dark Matter Experiment (ADMX) | year=2019 | last1=Tanner | first1=David B. | last2=Sullivan | first2=Neil | osti=1508642 | s2cid=204183272 }}</ref> were done and it is taking new data, including at {{val|4.9|–|6.2|u=μeV}}. In December 2021 it excluded the range {{val|3.3|–|4.2|u=μeV}} for the KSVZ model.<ref>{{cite journal |display-authors=6 |last1=Bartram |first1=C. |last2=Braine |first2=T. |last3=Burns |first3=E. |last4=Cervantes |first4=R. |last5=Crisosto |first5=N. |last6=Du |first6=N. |last7=Korandla |first7=H. |last8=Leum |first8=G. |last9=Mohapatra |first9=P. |last10=Nitta |first10=T. |last11=Rosenberg |first11=L. J |last12=Rybka |first12=G. |last13=Yang |first13=J. |last14=Clarke |first14=John |last15=Siddiqi |first15=I. |last16=Agrawal |first16=A. |last17=Dixit |first17=A. V. |last18=Awida |first18=M. H. |last19=Chou |first19=A. S. |last20=Hollister |first20=M. |last21=Knirck |first21=S. |last22=Sonnenschein |first22=A. |last23=Wester |first23=W. |last24=Gleason |first24=J. R. |last25=Hipp |first25=A. T. |last26=Jois |first26=S. |last27=Sikivie |first27=P. |last28=Sullivan |first28=N. S. |last29=Tanner |first29=D. B. |last30=Lentz |first30=E. |last31=Khatiwada |first31=R. |last32=Carosi |first32=G. |last33=Robertson |first33=N. |last34=Woollett |first34=N. |last35=Duffy |first35=L. D. |last36=Boutan |first36=C. |last37=Jones |first37=M. |last38=LaRoque |first38=B. H. |last39=Oblath |first39=N. S. |last40=Taubman |first40=M. S. |last41=Daw |first41=E. J. |last42=Perry |first42=M. G. |last43=Buckley |first43=J. H. |last44=Gaikwad |first44=C. |last45=Hoffman |first45=J. |last46=Murch |first46=K. W. |last47=Goryachev |first47=M. |last48=McAllister |first48=B. T. |last49=Quiskamp |first49=A. |last50=Thomson |first50=C. |last51=Tobar |first51=M. E. |title=Search for Invisible Axion Dark Matter in the 3.3 – 4.2 μ eV Mass Range |journal=Physical Review Letters |date=23 December 2021 |volume=127 |issue=26 |page=261803 |doi=10.1103/PhysRevLett.127.261803 |pmid=35029490 |bibcode=2021PhRvL.127z1803B |s2cid=238634307 |doi-access=free|arxiv=2110.06096 }}</ref><ref>{{cite journal |last1=Stephens |first1=Marric |title=Tightening the Net on Two Kinds of Dark Matter |journal=Physics |date=23 December 2021 |volume=14 |doi=10.1103/Physics.14.s164 |bibcode=2021PhyOJ..14.s164S |s2cid=247277808 |doi-access=free}}</ref> Other experiments of this type include DMRadio,<ref>{{cite journal |last1=Silva-Feaver |first1=Maximiliano |last2=Chaudhuri |first2=Saptarshi |last3=Cho |first3=Hsaio-Mei |last4=Dawson |first4=Carl |last5=Graham |first5=Peter |last6=Irwin |first6=Kent |last7=Kuenstner |first7=Stephen |last8=Li |first8=Dale |last9=Mardon |first9=Jeremy |last10=Moseley |first10=Harvey |last11=Mule |first11=Richard |last12=Phipps |first12=Arran |last13=Rajendran |first13=Surjeet |last14=Steffen |first14=Zach |last15=Young |first15=Betty |title=Design Overview of DM Radio Pathfinder Experiment |journal=IEEE Transactions on Applied Superconductivity |date=June 2017 |volume=27 |issue=4 |pages=1–4 |doi=10.1109/TASC.2016.2631425 |arxiv=1610.09344 |bibcode=2017ITAS...2731425S |s2cid=29416513 }}</ref> HAYSTAC,<ref name=HAYSTAC>{{cite journal |display-authors=6 |last1=Brubaker |first1=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 μ eV |journal=Physical Review Letters |date=9 February 2017 |volume=118 |issue=6 |page=061302 |doi=10.1103/physrevlett.118.061302 |s2cid=6509874 |pmid=28234529 |arxiv=1610.02580 |bibcode=2017PhRvL.118f1302B }} </ref> CULTASK,<ref name=CULTASK>{{cite journal |last1=Petrakou |first1=Eleni |title=Haloscope searches for dark matter axions at the Center for Axion and Precision Physics Research |journal=EPJ Web of Conferences |date=2017 |volume=164 |page=01012 |doi=10.1051/epjconf/201716401012 |s2cid=119381143 |arxiv=1702.03664 |bibcode=2017EPJWC.16401012P |url=https://inspirehep.net/record/1513138 }} </ref> and ORGAN.<ref name=ORGAN>{{cite journal |last1=McAllister |first1=Ben T. |last2=Flower |first2=Graeme |last3=Ivanov |first3=Eugene N. |last4=Goryachev |first4=Maxim |last5=Bourhill |first5=Jeremy |last6=Tobar |first6=Michael E. |title=The ORGAN experiment: An axion haloscope above 15 GHz |journal=Physics of the Dark Universe |date=December 2017 |volume=18 |pages=67–72 |doi=10.1016/j.dark.2017.09.010 |bibcode=2017PDU....18...67M |arxiv=1706.00209 |s2cid=118887710 }} </ref> HAYSTAC completed the first scanning run of a haloscope above 20 μeV in the late 2010s.<ref name=HAYSTAC/> Another type of direct conversion experiments are the [[Helioscope|helioscopes]] were the magnet is pointed at the Sun. Axions produced in the Sun would have an energy range of 1-10 keV and can therefore be converted into X-rays of the same energy in the magnet. The current state-of-the-art experiment is the [[CERN Axion Solar Telescope|CERN Axion Solar Telescope (CAST)]] which reached the axion-photon coupling limit of <math>5.8 \times 10^{-11} \ GeV^{-1}</math> at 95% CL (for <math>m_a</math> ≲ 0.02 eV) in 2024.<ref>{{Cite journal |last=CAST Collaboration |last2=Altenmüller |first2=K. |last3=Anastassopoulos |first3=V. |last4=Arguedas-Cuendis |first4=S. |last5=Aune |first5=S. |last6=Baier |first6=J. |last7=Barth |first7=K. |last8=Bräuninger |first8=H. |last9=Cantatore |first9=G. |last10=Caspers |first10=F. |last11=Castel |first11=J. F. |last12=Çetin |first12=S. A. |last13=Christensen |first13=F. |last14=Cogollos |first14=C. |last15=Dafni |first15=T. |date=2024-11-27 |title=New Upper Limit on the Axion-Photon Coupling with an Extended CAST Run with a Xe-Based Micromegas Detector |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.221005 |journal=Physical Review Letters |volume=133 |issue=22 |pages=221005 |doi=10.1103/PhysRevLett.133.221005}}</ref> The next generation helioscope is the [[International Axion Observatory|International AXion Observatory (IAXO)]] which is currently in development. === Polarized light in a magnetic field === 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. |author-link1=Luciano Maiani |last2=Petronzio |first2=R. |last3=Zavattini |first3=E. |author-link3=Emilio Zavattini |date=7 August 1986 |title=Effects of nearly massless, spin-zero particles on light propagation in a magnetic field |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 |url=http://cdsweb.cern.ch/record/167556/files/198606014.pdf }} </ref> A rotation claim<ref> {{cite magazine |first1=Steve |last1=Reucroft |first2=John |last2=Swain |date=2006-10-05 |title=Axion signature may be QED |magazine=CERN Courier |url=http://cerncourier.com/main/article/46/8/10 |archive-url=https://web.archive.org/web/20080820154706/http://cerncourier.com/main/article/46/8/10 |archive-date=2008-08-20 |df=dmy-all }} </ref> in 2006 was excluded by an upgraded setup.<ref name="zavattini_1"> {{cite journal |last1=Zavattini |first1=E. |collaboration=PVLAS Collaboration |display-authors=etal |year=2006 |title=Experimental Observation of Optical Rotation Generated in Vacuum by a Magnetic Field |journal=Physical Review Letters |volume=96 |issue=11 |page=110406 |doi=10.1103/PhysRevLett.96.110406 |pmid=16605804 |arxiv=hep-ex/0507107 |bibcode=2006PhRvL..96k0406Z }}</ref> An optimized search began in 2014. === Light shining through walls === Another technique is so called "light shining through walls",<ref> {{cite conference |last=Ringwald |first=A. |date=16–21 October 2001 |title=Fundamental Physics at an X-Ray Free Electron Laser |book-title=Electromagnetic Probes of Fundamental Physics – Proceedings of the Workshop |pages=63–74 |conference=Workshop on Electromagnetic Probes of Fundamental Physics |location=[[Erice]], Italy |arxiv=hep-ph/0112254 |doi=10.1142/9789812704214_0007 |isbn=978-981-238-566-6 }}</ref> 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.<ref name="rizzo_1"> {{cite journal |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. |year=2007 |title=No 'light shining through a wall': Results from a photoregeneration experiment |journal=Physical Review Letters |volume=99 |issue=19 |page=190403 |arxiv=0707.1296 |doi=10.1103/PhysRevLett.99.190403 |pmid=18233050 |bibcode=2007PhRvL..99s0403R |s2cid=23159010 }} </ref> GammeV saw no events, reported in a 2008 Physics Review Letter. ALPS I conducted similar runs,<ref> {{cite journal |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 |display-authors=6 |title=New ALPS results on hidden-sector lightweights |date=May 2010 |journal= Physics Letters B |volume=689 |issue=4–5 |pages=149–155 |doi=10.1016/j.physletb.2010.04.066 |bibcode=2010PhLB..689..149E |arxiv=1004.1313 |s2cid=58898031 |url=http://pubman.mpdl.mpg.de/pubman/faces/viewItemOverviewPage.jsp?itemId=escidoc:417635 }}</ref> setting new constraints in 2010; ALPS II began collecting data in May 2023.<ref>{{cite journal |last1=Diaz Ortiz |first1=M. |last2=Gleason |first2=J. |last3=Grote |first3=H. |last4=Hallal |first4=A. |last5=Hartman |first5=M.T. |last6=Hollis |first6=H. |last7=Isleif |first7=K.-S. |last8=James |first8=A. |last9=Karan |first9=K. |last10=Kozlowski |first10=T. |last11=Lindner |first11=A. |last12=Messineo |first12=G. |last13=Mueller |first13=G. |last14=Põld |first14=J.H. |last15=Smith |first15=R.C.G. |last16=Spector |first16=A.D. |last17=Tanner |first17=D.B. |last18=Wei |first18=L.-W. |last19=Willke |first19=B. |title=Design of the ALPS II optical system |journal=Physics of the Dark Universe |date=March 2022 |volume=35 |pages=100968 |doi=10.1016/j.dark.2022.100968 |bibcode=2022PDU....3500968D |s2cid=222067049 |doi-access=free |arxiv=2009.14294 }}</ref><ref>{{cite web |date=2023-05-23 |title='Light shining through a wall' experiment ALPS starts searching for dark matter |url=https://www.desy.de/news/news_search/index_eng.html?openDirectAnchor=2758 |access-date=2024-09-25 |website=DESY |language=en}}</ref> OSQAR found no signal, limiting coupling,<ref> {{cite journal |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. |display-authors=6 |date=August 2014 |title=Search for weakly interacting sub-eV particles with the OSQAR laser-based experiment: Results and perspectives |journal=The European Physical Journal C |volume=74 |issue=8 |pages=3027 |doi=10.1140/epjc/s10052-014-3027-8 |bibcode=2014EPJC...74.3027P |arxiv=1306.0443|s2cid=29889038 }}</ref> and will continue. === Astrophysical axion searches === 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 ([[Very-high-energy gamma ray|very-high-energy gamma rays]]).<ref> {{cite journal |last1=De Angelis |first1=A. |last2=Mansutti |first2=O. |last3=Roncadelli |first3=M. |year=2007 |title=Evidence for a new light spin-zero boson from cosmological gamma-ray propagation? |journal=Physical Review D |volume=76 |issue=12 |page=121301 |bibcode=2007PhRvD..76l1301D |s2cid=119152884 |doi=10.1103/PhysRevD.76.121301 |arxiv=0707.4312 }} </ref><ref> {{cite journal |last1=De Angelis |first1=A. |last2=Mansutti |first2=O. |last3=Persic |first3=M. |last4=Roncadelli |first4=M. |year=2009 |title=Photon propagation and the very high energy gamma-ray spectra of blazars: How transparent is the Universe? |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=394 |issue=1 |pages=L21–L25 |doi=10.1111/j.1745-3933.2008.00602.x |doi-access=free |arxiv=0807.4246 |bibcode=2009MNRAS.394L..21D |s2cid=18184567 }} </ref> It has also been demonstrated 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 early 21st century telescopes.<ref> {{cite journal |last1=Chelouche |first1=Doron |last2=Rabadan |first2=Raul |last3=Pavlov |first3=Sergey S. |last4 =Castejon |first4=Francisco |year=2009 |title=Spectral signatures of photon–particle oscillations from celestial objects |journal=The Astrophysical Journal |series=Supplement Series |volume=180 |issue=1 |pages=1–29 |arxiv=0806.0411 |doi=10.1088/0067-0049/180/1/1 |s2cid=5018245 |bibcode=2009ApJS..180....1C }} </ref> 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.<ref> {{cite journal |last1=Chelouche |first1=Doron |last2=Guendelman |first2=Eduardo I. |year=2009 |title=Cosmic analogs of the Stern–Gerlach experiment and the detection of light bosons |journal=The Astrophysical Journal |volume=699 |issue=1 |pages=L5–L8 |doi=10.1088/0004-637X/699/1/L5 |arxiv=0810.3002 |bibcode=2009ApJ...699L...5C |s2cid=11868951 }} </ref> The [[International Axion Observatory]] (IAXO) is a proposed fourth generation [[helioscope]].<ref> {{cite web |title=The International Axion Observatory |publisher=[[CERN]] |url=http://iaxo.web.cern.ch/content/home-international-axion-observatory |access-date=19 March 2016 }} </ref> Axions can resonantly convert into photons in the [[magnetosphere]]s of [[neutron star]]s.<ref> {{cite journal |last1=Pshirkov |first1=Maxim S. |last2=Popov |first2=Sergei B. |year=2009 |title=Conversion of Dark matter axions to photons in magnetospheres of neutron stars |journal=Journal of Experimental and Theoretical Physics |volume= 108 |issue=3 |pages=384–388 |doi=10.1134/S1063776109030030 |arxiv=0711.1264 |bibcode= 2009JETP..108..384P |s2cid=119269835 }} </ref> 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 mass range {{val|5|–|11|u=μeV/''c''<sup>2</sup>}}, by re-analyzing existing data from the [[Green Bank Telescope]] and the [[Effelsberg 100-m Radio Telescope|Effelsberg 100 m Radio Telescope]].<ref> {{cite journal |last1=Foster |first1=Joshua W. |last2=Kahn |first2=Yonatan |last3=Macias |first3=Oscar |last4=Sun |first4=Zhiquan |last5=Eatough |first5=Ralph P. |last6=Kondratiev |first6=Vladislav I. |last7=Peters |first7=Wendy M. |last8=Weniger |first8=Christoph |last9=Safdi |first9=Benjamin R. |year=2020 |title=Green Bank and Effelsberg Radio Telescope Searches for Axion Dark Matter Conversion in Neutron Star Magnetospheres |journal=Physical Review Letters |volume=125 |number=17 |pages= 171301 |doi=10.1103/PhysRevLett.125.171301 |pmid=33156637 |arxiv=2004.00011 |bibcode= 2020PhRvL.125q1301F |s2cid=214743261 }} </ref> 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]].<ref> {{cite journal |last1=Edwards |first1=Thomas D. P. |last2=Kavanagh |first2=Bradley J. |last3=Visinelli |first3=Luca |last4=Weniger |first4=Christoph |year=2021 |title=Transient Radio Signatures from Neutron Star Encounters with QCD Axion Miniclusters |journal=Physical Review Letters |volume=127 |number=13 |pages= 131103 |doi=10.1103/PhysRevLett.127.131103 |pmid=34623827 |arxiv=2011.05378 |bibcode=2021PhRvL.127m1103E |s2cid=226300099 }} </ref> 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. Axions may also 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 Gamma-ray Space Telescope]]. From an analysis of four neutron stars, Berenji et al. (2016) obtained a 95% [[confidence interval]] upper limit on the axion mass of {{val|0.079|u=eV/c2}}.<ref> {{cite journal |last1=Berenji |first1=B. |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 |bibcode=2016PhRvD..93d5019B |s2cid=118723146 }} </ref> In 2021 it has been also suggested<ref>{{cite journal |last1=Buschmann |first1=Malte |last2=Co |first2=Raymond T. |last3=Dessert |first3=Christopher |last4=Safdi |first4=Benjamin R. |title=Axion Emission Can Explain a New Hard X-Ray Excess from Nearby Isolated Neutron Stars |journal=Physical Review Letters |date=12 January 2021 |volume=126 |issue=2 |page=021102 |doi=10.1103/PhysRevLett.126.021102 |pmid=33512228 |arxiv=1910.04164 |bibcode=2021PhRvL.126b1102B |s2cid=231764983 }}</ref><ref>{{cite web |last=O'Callaghan |first=Jonathan |date=2021-10-19 |title=A Hint of Dark Matter Sends Physicists Looking to the Skies |url=https://www.quantamagazine.org/a-hint-of-dark-matter-sends-physicists-looking-to-the-skies-20211019/ |access-date=2021-10-25 |website=Quanta Magazine |language=en }}</ref> that a reported<ref>{{cite journal |last1=Dessert |first1=Christopher |last2=Foster |first2=Joshua W. |last3=Safdi |first3=Benjamin R. |title=Hard X-Ray Excess from the Magnificent Seven Neutron Stars |journal=The Astrophysical Journal |date=November 2020 |volume=904 |issue=1 |pages=42 |doi=10.3847/1538-4357/abb4ea |arxiv=1910.02956 |bibcode=2020ApJ...904...42D |s2cid=203902766 |doi-access=free }}</ref> excess of hard X-ray emission from a system of neutron stars known as the [[The Magnificent Seven (neutron stars)|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 [[Magnetic resonance imaging|MRI]] scanning machine. It would show variation, a slight wavering, that is linked to the mass of the axion. Results from the ensuing experiment published in 2021 reported no evidence of axions in the mass range from 4.1x10<sup>−10</sup> to 8.27x10<sup>−9</sup> eV.<ref>{{cite journal |last1=Salemi |first1=Chiara P. |last2=Foster |first2=Joshua W. |last3=Ouellet |first3=Jonathan L. |last4=Gavin |first4=Andrew |last5=Pappas |first5=Kaliroë M. W. |last6=Cheng |first6=Sabrina |last7=Richardson |first7=Kate A. |last8=Henning |first8=Reyco |last9=Kahn |first9=Yonatan |last10=Nguyen |first10=Rachel |last11=Rodd |first11=Nicholas L. |last12=Safdi |first12=Benjamin R. |last13=Winslow |first13=Lindley |date=2021-08-17 |title=Search for Low-Mass Axion Dark Matter with ABRACADABRA-10 cm |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.081801 |journal=Physical Review Letters |volume=127 |issue=8 |pages=081801 |doi=10.1103/PhysRevLett.127.081801|pmid=34477408 |arxiv=2102.06722 |bibcode=2021PhRvL.127h1801S }}</ref> In 2022 the polarized light [[Event Horizon Telescope#Messier 87*|measurements]] of [[Messier 87|Messier 87*]] by the [[Event Horizon Telescope]] were used to constrain the mass of the axion assuming that hypothetical clouds of axions could form around a black hole, rejecting the approximate {{val|e=-21|u=eV/c2}} – {{val|e=-20|u=eV/c2}} range of mass values.<ref>{{cite journal |last1=Chen |first1=Yifan |last2=Liu |first2=Yuxin |last3=Lu |first3=Ru-Sen |last4=Mizuno |first4=Yosuke |last5=Shu |first5=Jing |last6=Xue |first6=Xiao |last7=Yuan |first7=Qiang |last8=Zhao |first8=Yue |title=Stringent axion constraints with Event Horizon Telescope polarimetric measurements of M87⋆ |journal=Nature Astronomy |date=17 March 2022 |volume=6 |issue=5 |pages=592–598 |doi=10.1038/s41550-022-01620-3 |arxiv=2105.04572 |bibcode=2022NatAs...6..592C |s2cid=247188135 }}</ref><ref>{{cite news |last1=Kruesi |first1=Liz |title=How light from black holes is narrowing the search for axions |url=https://www.sciencenews.org/article/black-hole-light-axion-particle-search-event-horizon |work=Science News |date=17 March 2022 }}</ref> === Searches for resonance effects === Resonance effects may be evident in [[Josephson junction]]s<ref> {{cite journal |last=Beck |first=Christian |date=2 December 2013 |title=Possible Resonance Effect of Axionic Dark Matter in Josephson Junctions |journal=Physical Review Letters |volume=111 |issue=23 |page=1801 |doi=10.1103/PhysRevLett.111.231801 |pmid=24476255 |s2cid=23845250 |arxiv=1309.3790 |bibcode=2013PhRvL.111w1801B }} </ref> from a supposed high flux of axions from the galactic halo with mass of {{val|110|u=μeV/''c''<sup>2</sup>}} and density {{val|0.05|u=(GeV/''c''<sup>2</sup>)/cm<sup>3</sup>}}<ref> {{cite magazine |last=Moskvitch |first=Katia |title=Hints of cold dark matter pop up in 10 year-old circuit |magazine=[[New Scientist]] Magazine |url=https://www.newscientist.com/article/dn24689-hints-of-cold-dark-matter-pop-up-in-10yearold-circuit.html |access-date=3 December 2013 }} </ref> compared to the implied dark matter density {{val|0.3|0.1|u=(GeV/''c''<sup>2</sup>)/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 recoil searches === 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-day run to set the best coupling limits to date and exclude some parameters.<ref> {{cite journal |first1=E. |last1=Aprile |display-authors=etal |date=9 September 2014 |title=First axion results from the XENON100 experiment |journal=Physical Review D |volume=90 |issue=6 |pages=062009 |doi=10.1103/PhysRevD.90.062009 |s2cid=55875111 |bibcode=2014PhRvD..90f2009A |arxiv=1404.1455 |url=https://science.purdue.edu/xenon1t/?p=292 }} </ref> === Nuclear spin precession === While Schiff's theorem states that a static nuclear electric dipole moment (EDM) does not produce atomic and molecular EDMs,<ref>{{cite journal |last1=Commins |first1=Eugene D. |last2=Jackson |first2=J. D. |last3=DeMille |first3=David P. |title=The electric dipole moment of the electron: An intuitive explanation for the evasion of Schiff's theorem |journal=American Journal of Physics |date=June 2007 |volume=75 |issue=6 |pages=532–536 |doi=10.1119/1.2710486 |bibcode=2007AmJPh..75..532C }}</ref> the axion induces an oscillating nuclear EDM that oscillates at the [[Larmor precession|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.<ref>{{cite journal |last1=Flambaum |first1=V. V. |last2=Tan |first2=H. B. Tran |title=Oscillating nuclear electric dipole moment induced by axion dark matter produces atomic and molecular electric dipole moments and nuclear spin rotation |journal=Physical Review D |date=27 December 2019 |volume=100 |issue=11 |page=111301 |doi=10.1103/PhysRevD.100.111301 |arxiv=1904.07609 |s2cid=119303702 |bibcode=2019PhRvD.100k1301F }}</ref> An experiment using this technique is the Cosmic Axion Spin Precession Experiment (CASPEr).<ref>{{cite journal |last1=Budker |first1=Dmitry |last2=Graham |first2=Peter W. |last3=Ledbetter |first3=Micah |last4=Rajendran |first4=Surjeet |last5=Sushkov |first5=Alexander O. |title=Proposal for a Cosmic Axion Spin Precession Experiment (CASPEr) |journal=Physical Review X |date=19 May 2014 |volume=4 |issue=2 |page=021030 |doi=10.1103/PhysRevX.4.021030 |arxiv=1306.6089 |bibcode=2014PhRvX...4b1030B |s2cid=118351193 }}</ref><ref>{{cite journal |display-authors=6 |last1=Garcon |first1=Antoine |last2=Aybas |first2=Deniz |last3=Blanchard |first3=John W |last4=Centers |first4=Gary |last5=Figueroa |first5=Nataniel L |last6=Graham |first6=Peter W |last7=Kimball |first7=Derek F Jackson |last8=Rajendran |first8=Surjeet |last9=Sendra |first9=Marina Gil |last10=Sushkov |first10=Alexander O |last11=Trahms |first11=Lutz |last12=Wang |first12=Tao |last13=Wickenbrock |first13=Arne |last14=Wu |first14=Teng |last15=Budker |first15=Dmitry |title=The cosmic axion spin precession experiment (CASPEr): a dark-matter search with nuclear magnetic resonance |journal=Quantum Science and Technology |date=January 2018 |volume=3 |issue=1 |pages=014008 |doi=10.1088/2058-9565/aa9861 |arxiv=1707.05312 |bibcode=2018QS&T....3a4008G |s2cid=51686418 }}</ref><ref>{{cite journal |display-authors=6 |last1=Aybas |first1=Deniz |last2=Adam |first2=Janos |last3=Blumenthal |first3=Emmy |last4=Gramolin |first4=Alexander V. |last5=Johnson |first5=Dorian |last6=Kleyheeg |first6=Annalies |last7=Afach |first7=Samer |last8=Blanchard |first8=John W. |last9=Centers |first9=Gary P. |last10=Garcon |first10=Antoine |last11=Engler |first11=Martin |last12=Figueroa |first12=Nataniel L. |last13=Sendra |first13=Marina Gil |last14=Wickenbrock |first14=Arne |last15=Lawson |first15=Matthew |last16=Wang |first16=Tao |last17=Wu |first17=Teng |last18=Luo |first18=Haosu |last19=Mani |first19=Hamdi |last20=Mauskopf |first20=Philip |last21=Graham |first21=Peter W. |last22=Rajendran |first22=Surjeet |last23=Kimball |first23=Derek F. Jackson |last24=Budker |first24=Dmitry |last25=Sushkov |first25=Alexander O. |title=Search for Axionlike Dark Matter Using Solid-State Nuclear Magnetic Resonance |journal=Physical Review Letters |date=9 April 2021 |volume=126 |issue=14 |page=141802 |doi=10.1103/PhysRevLett.126.141802 |pmid=33891466 |arxiv=2101.01241 |bibcode=2021PhRvL.126n1802A |s2cid=230524028 }}</ref> === Searches at particle colliders === 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 [[Two-photon physics|light-by-light scattering]] process. Those searches are sensitive for rather large axion masses between {{val|100|u=MeV/c2}} and hundreds of {{val|u=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.<ref>{{cite journal |last1=Bauer |first1=Martin |last2=Neubert |first2=Matthias |last3=Thamm |first3=Andrea |date=December 2017 |title=Collider Probes of Axion-Like Particles |journal=Journal of High Energy Physics |volume=2017 |issue=12 |page=44 |doi=10.1007/JHEP12(2017)044 |arxiv=1708.00443 |bibcode=2017JHEP...12..044B |s2cid=119422560 }}</ref>
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