Editing Muon (section)

== Anomalous magnetic dipole moment ==
The [[anomalous magnetic dipole moment]] is the difference between the experimentally observed value of the magnetic dipole moment and the theoretical value predicted by the [[Dirac equation]]. The measurement and prediction of this value is very important in the [[precision tests of QED]]. The E821 experiment<ref>{{cite web |url=http://www.g-2.bnl.gov/ |title=The Muon g-2 Experiment Home Page |publisher=G-2.bnl.gov |date=2004-01-08 |access-date=2012-01-06 |df=dmy-all}}</ref> at Brookhaven and the [[Muon g-2]] experiment at Fermilab studied the precession of the muon spin in a constant external magnetic field as the muons circulated in a confining storage ring. The Muon ''g''−2 collaboration reported<ref>{{cite journal| title=Measurement of the Positive Muon Magnetic Moment to 0.46 ppm | last1=Abi | first1=B. | last2=Albahri | first2=T. | last3=Al-Kilani | first3=S. | last4=Allspach | first4=D. | last5=Alonzi | first5=L.P. | last6=Anastasi | first6=A. | last7=Anisenkov | first7=A. | display-authors=6 | journal = Phys Rev Lett | year=2021 | volume=126 | issue=14 | pages=141801 | doi=10.1103/PhysRevLett.126.141801| arxiv=2104.03281| pmid=33891447 | bibcode=2021PhRvL.126n1801A | doi-access=free }}</ref> in 2021:
: <math>a = \frac{g-2}{2} = 0.00116592061(41).</math>

The prediction for the value of the muon anomalous magnetic moment includes three parts:
: {{mvar|a}}<sub>''μ''</sub><sup>SM</sup> = {{mvar|a}}<sub>''μ''</sub><sup>QED</sup> + {{mvar|a}}<sub>''μ''</sub><sup>EW</sup> + {{mvar|a}}<sub>''μ''</sub><sup>had</sup>.

The difference between the [[g-factor (physics)|''g''-factor]]s of the muon and the electron is due to their difference in mass. Because of the muon's larger mass, contributions to the theoretical calculation of its anomalous magnetic dipole moment from [[Standard Model]] [[weak interaction]]s and from contributions involving [[hadron]]s are important at the current level of precision, whereas these effects are not important for the electron. The muon's anomalous magnetic dipole moment is also sensitive to contributions from new physics [[beyond the Standard Model]], such as [[supersymmetry]]. For this reason, the muon's anomalous magnetic moment is normally used as a probe for new physics beyond the Standard Model rather than as a test of [[Quantum electrodynamics|QED]].<ref>{{cite journal |doi=10.1016/j.physletb.2007.04.012| arxiv=hep-ph/0611102 |last1=Hagiwara |first1=K |last2=Martin |first2=A |last3=Nomura |first3=D |last4=Teubner |first4=T |title=Improved predictions for g−2 of the muon and {{mvar|α}}{{sup|QED}}(MZ2) |journal=Physics Letters&nbsp;B |volume=649 |issue=2–3 |pages=173–179 |year=2007 |bibcode = 2007PhLB..649..173H |s2cid=118565052 }}</ref> [[Muon g−2|Muon&nbsp;''g''−2]], a new experiment at Fermilab using the E821 magnet improved the precision of this measurement.<ref>{{cite press release |url = http://www.fnal.gov/pub/presspass/press_releases/2013/Muon-g-2-201305.html |title = Revolutionary muon experiment to begin with 3,200&nbsp;mile move of 50&nbsp;foot-wide particle storage ring |date = 8 May 2013 |access-date = 16 March 2015}}</ref>

In 2020 an international team of 170 physicists calculated the most accurate prediction for the theoretical value of the muon's anomalous magnetic moment.<ref>{{cite web |last1=Pinson |first1=Jerald |title=Physicists publish worldwide consensus of muon magnetic moment calculation |url=https://news.fnal.gov/2020/06/physicists-publish-worldwide-consensus-of-muon-magnetic-moment-calculation/ |website=Fermilab News |access-date=13 February 2022 |date=11 June 2020}}</ref><ref>{{cite journal |last1=Aoyama |first1=T. |display-authors=etal |title=The anomalous magnetic moment of the muon in the Standard Model |journal=Physics Reports |date=December 2020 |volume=887 |pages=1–166 |doi=10.1016/j.physrep.2020.07.006|arxiv=2006.04822 |bibcode=2020PhR...887....1A |s2cid=219559166 }}</ref>

=== Muon g−2 ===
[[Muon g-2]] is a particle physics experiment at Fermilab to measure the anomalous magnetic dipole moment of a muon to a precision of 0.14 ppm,<ref>{{cite web |title=Muon {{mvar|g}} − 2 Experiment |type=main page |publisher=Fermilab |lang=en |url=http://muon-g-2.fnal.gov/ |access-date=April 26, 2017}}</ref><ref name="NYT-20230810">{{cite news |last=Miller |first=Katrina |title=Physicists Move One Step Closer to a Theoretical Showdown - The deviance of a tiny particle called the muon might prove that one of the most well-tested theories in physics is incomplete. + comment |url=https://www.nytimes.com/2023/08/10/science/physics-muons-g2-fermilab.html |date=10 August 2023 |work=[[The New York Times]] |url-status=bot: unknown |archiveurl=https://web.archive.org/web/20230811000933/https://www.nytimes.com/2023/08/10/science/physics-muons-g2-fermilab.html#permid=126945707 |archivedate=11 August 2023 |accessdate=11 August 2023 }}</ref> which is a sensitive test of the Standard Model.<ref>{{cite journal |last1=Keshavarzi |first1=Alex |last2=Khaw |first2=Kim Siang |last3=Yoshioka |first3=Tamaki |date=2022-01-22 |title=Muon g − 2: A review |url=https://linkinghub.elsevier.com/retrieve/pii/S0550321322000268 |journal=Nuclear Physics B |language=en |volume=975 |pages=115675 |doi=10.1016/j.nuclphysb.2022.115675|arxiv=2106.06723 |bibcode=2022NuPhB.97515675K |s2cid=245880824 }}</ref> It might also provide evidence of the existence of entirely new particles.<ref>{{cite journal |last=Gibney |first=Elizabeth |date=April 13, 2017 |title=Muons' big moment could fuel new physics |journal=Nature |lang=en |volume=544 |issue=7649 |pages=145–146 |doi=10.1038/544145a |pmid=28406224 |bibcode=2017Natur.544..145G|s2cid=4400589 |doi-access=free }}</ref>

In 2021, the Muon ''g''−2 Experiment presented their first results of a new experimental average that increased the difference between experiment and theory to 4.2 standard deviations.<ref>{{cite journal |last1=Abi |first1=B. |display-authors=etal |collaboration=Muon ''g''−2 Collaboration|title=Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm |journal=Physical Review Letters |date=7 April 2021 |volume=126 |issue=14 |pages=141801 |doi=10.1103/PhysRevLett.126.141801|pmid=33891447 |arxiv=2104.03281 |bibcode=2021PhRvL.126n1801A |s2cid=233169085 }}</ref>