View source for Muonium

{{short description|Type of atom}}
{{for multi|atoms where muons have replaced one or more electrons|Muonic atom|the onium of a muon and an antimuon|True muonium}}

[[File:Muonium.svg|alt=Simplified drawing of the muonium atom|thumb|300x300px|A muonium atom]]

{{Antimatter}}

'''Muonium''' ({{IPAc-en|m|juː|.|ˈ|oʊ|.|n|i|.|ə|m}}, {{respell|mew|OH|nee|əm}}) is an [[exotic atom]] made up of an [[Muon|antimuon]] and an [[electron]],<ref name="Gold">{{cite book |author=IUPAC |author-link=International Union of Pure and Applied Chemistry |year=1997 |editor=A.D. McNaught, A. Wilkinson |chapter=Muonium |chapter-url=http://goldbook.iupac.org/M04069.html |title=Compendium of Chemical Terminology |edition=2nd |publisher=[[Blackwell Scientific Publications]] |isbn=978-0-86542-684-9 |doi=10.1351/goldbook.M04069 |title-link=Compendium of Chemical Terminology }}</ref> which was discovered in 1960 by [[Vernon W. Hughes]]<ref name="Hughes">{{cite journal |author=V.W. Hughes|year=1960 | author-link =Vernon W. Hughes  |title=Formation of Muonium and Observation of its Larmor Precession |journal=[[Physical Review Letters]] |volume=5 |issue=2 |pages=63&ndash;65 |doi=10.1103/PhysRevLett.5.63 |bibcode=1960PhRvL...5...63H |display-authors=etal }}</ref> and is given the chemical symbol Mu. During the muon's {{val|2.2|u=[[Microsecond|µs]]}} lifetime, muonium can undergo chemical reactions.<ref name="iupac">{{cite journal |doi=10.1351/pac200173020377 |author=W.H. Koppenol ([[International Union of Pure and Applied Chemistry|IUPAC]]) |year=2001 |title=Names for muonium and hydrogen atoms and their ions |url=http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf |journal=[[Pure and Applied Chemistry]] |volume=73 |issue=2 |pages=377–380|s2cid=97138983 }}</ref>

==Description==
Because, like a proton, the antimuon's mass is much larger than that of the electron, muonium ({{SubatomicParticle|Antimuon}}{{SubatomicParticle|Electron}}) is more similar to [[hydrogen atom|atomic hydrogen]] ({{SubatomicParticle|Proton+}}{{SubatomicParticle|Electron}}) than [[positronium]] ({{SubatomicParticle|Positron}}{{SubatomicParticle|Electron}}). Its [[Bohr radius]] and ionization energy are within 0.5% of [[hydrogen]], [[deuterium]], and [[tritium]], and thus it can usefully be considered as an exotic light isotope of hydrogen.<ref>{{cite book | page=4 | url=https://books.google.com/books?id=PM88AAAAIAAJ | title=Muon and Muonium Chemistry | isbn=978-0-521-24241-7 | last1=Walker | first1=David C | date=1983-09-08| publisher=Cambridge University Press }}</ref>

==Properties==
Although muonium is short-lived, physical chemists study it using [[muon spin spectroscopy]] (μSR),<ref name="Brewer">{{cite journal |author=J.H. Brewer |year=1994 |title=Muon Spin Rotation/Relaxation/Resonance |journal=Encyclopedia of Applied Physics |volume=11 |pages=23&ndash;53 }}</ref> a magnetic resonance technique analogous to [[nuclear magnetic resonance]] (NMR) or [[electron spin resonance]] (ESR) [[spectroscopy]]. Like ESR, μSR is useful for the analysis of chemical transformations and the structure of compounds with novel or potentially valuable electronic properties. Muonium is usually studied by [[muon spin rotation]], in which the muonium atom's spin precesses in a [[magnetic field]] applied transverse to the muon spin direction (since muons are typically produced in a [[Spin polarization|spin-polarized]] state from the decay of [[pion]]s), and by [[Avoided crossing|avoided level crossing]] (ALC), which is also called [[Resonance|level crossing resonance]] (LCR).<ref name="Brewer" /> The latter employs a magnetic field applied longitudinally to the polarization direction, and monitors the relaxation of muon spins caused by "flip/flop" transitions with other magnetic nuclei.

Because the muon is a [[lepton]], the atomic energy levels of muonium can be calculated with great precision from [[quantum electrodynamics]] (QED), unlike in the case of hydrogen, where the precision is limited by uncertainties related to the internal structure of the [[proton]]. For this reason, muonium is an ideal system for studying bound-state QED and also for searching for physics beyond the [[Standard Model]].<ref name="Jungmann">{{cite journal |author=K.P. Jungmann |year=2004 |title=Past, Present and Future of Muonium |arxiv=nucl-ex/0404013 |journal=Proceedings of the Memorial Symposium in Honor of Vernon Willard Hughes, New Haven, Connecticut, 14–15 Nov 2003 |bibcode=2004shvw.conf..134J |doi=10.1142/9789812702425_0009 |isbn=978-981-256-050-6 |pages=134–153 |citeseerx=10.1.1.261.4459 |s2cid=16164836 }}</ref><ref name="Arrell">{{cite web |url=https://phys.org/news/2022-11-muonium-reveal-physics-standard.html |title=Studying muonium to reveal new physics beyond the Standard Model |last=Arrell |first=Miriam |date=2022-11-29 |website=Phys.org |access-date=2023-01-06 }}</ref>

== Nomenclature ==
Normally in the nomenclature of particle physics, an atom composed of a positively charged particle bound to an electron is named after the positive particle with "-ium" replacing an "-on" suffix, in this case "muium".  Replacing "-on" with (or otherwise appending) "-[[onium]]" is mostly used for [[bound state]]s of a particle with its own antiparticle. The exotic atom consisting of a muon and an antimuon (which is yet to be observed) is known as [[true muonium]].{{Citation needed|date=July 2024}}

== See also ==
* [[Muon#Negative muon atoms|Muonic hydrogen]]
* [[Muon-catalyzed fusion]]

==References==
{{Reflist}}

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[[Category:Exotic atoms]]

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