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((Short description|Atoms composed of exotic particles))((Confuse|Exotic matter))((refimprove|date=December 2021))
An '''exotic atom''' is an otherwise normal [[atom]] in which one or more sub-atomic particles have been replaced by other particles of the same [[Electric charge|charge]]. For example, [[electron]]s may be replaced by other negatively charged particles such as [[muon]]s (muonic atoms) or [[pion]]s (pionic atoms).<ref>§1.8, ''Constituents of Matter: Atoms, Molecules, Nuclei and Particles'', Ludwig Bergmann, Clemens Schaefer, and Wilhelm Raith, Berlin: Walter de Gruyter, 1997, ((ISBN|3-11-013990-1)).</ref><ref name="as">[https://www.accessscience.com/content/exotic-atoms/YB000560 Exotic atoms] ((webarchive|url=https://web.archive.org/web/20071222044613/http://www.accessscience.com/abstract.aspx?id=YB000560 |date=2007-12-22)), AccessScience, McGraw-Hill. accessdate=September 26, 2007.</ref> Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and no exotic atom observed so far can persist under normal conditions.

== Muonic atoms ==
[[File:Hydrogen-4.1.svg|alt=Hydrogen 4.1 picture|thumb|Muonic helium, made out of 2 protons, 2 neutrons, 1 muon and 1 electron.]]
In a ''muonic atom'' (previously called a ''mu-mesic'' atom, now known to be a misnomer as muons are not [[meson]]s),<ref>[http://vega.org.uk/video/subseries/8 Dr. Richard Feynman's Douglas Robb Memorial Lectures]</ref> an electron is replaced by a muon, which, like the electron, is a [[lepton]]. Since [[lepton]]s are only sensitive to [[weak interaction|weak]], [[electromagnetic interaction|electromagnetic]] and [[gravity|gravitational]] forces, muonic atoms are governed to very high precision by the electromagnetic interaction.

Since a muon is more massive than an electron, the [[Bohr model|Bohr orbits]] are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to [[quantum electrodynamics]] are more important. Study of muonic atoms' [[energy level]]s as well as transition rates from [[excited state]]s to the [[ground state]] therefore provide experimental tests of quantum electrodynamics.

[[Muon-catalyzed fusion]] is a technical application of muonic atoms.

=== Muonic hydrogen ===
Muonic hydrogen is like normal hydrogen with the electron replaced by a negative muon—that is a proton orbited by a muon.  It is important in addressing the [[proton radius puzzle]].

=== Muonic helium (Hydrogen-4.1) ===
The symbol <sup>4.1</sup>H (Hydrogen-4.1) has been used to describe the exotic atom muonic helium (<sup>4</sup>He-μ), which is like [[helium-4]] in having two [[proton]]s and two [[neutron]]s.<ref>
((Cite journal
  | title = Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H<sub>2</sub>
  | journal = Science | volume = 331 | issue = 6016 | pages = 448–450 | date = 28 Jan 2011
  | doi = 10.1126/science.1199421 
  | last1 = Fleming | first1 = D. G.
  | last2 = Arseneau | first2 = D. J.
  | last3 = Sukhorukov | first3 = O.
  | last4 = Brewer | first4 = J. H.
  | last5 = Mielke | first5 = S. L.
  | last6 = Schatz | first6 = G. C.
  | last7 = Garrett | first7 = B. C.
  | last8 = Peterson | first8 = K. A.
  | last9 = Truhlar | first9 = D. G.
  | pmid = 21273484 | bibcode = 2011Sci...331..448F | s2cid = 206530683
 ))</ref> However one of its [[electron]]s is replaced by a [[muon]], which also has charge –1.  Because the muon's orbital radius is less than ((sfrac|1|200th)) the electron's orbital radius (due to the mass ratio), the muon can be considered as a part of the nucleus. The atom then has a [[atomic nucleus|nucleus]] with two protons, two neutrons and one muon, with total nuclear charge +1 (from two protons and one muon) and only one electron outside, so that it is effectively an isotope of hydrogen instead of an isotope of helium. A muon's weight is approximately 0.1 [[atomic mass unit|Da]] so the isotopic mass is 4.1. Since there is only one electron outside the nucleus, the hydrogen-4.1 atom can react with other atoms. Its chemical behavior is that of a hydrogen atom and not a noble helium atom.<ref>((cite web |title=The Muonic Helium Atom |url=https://www.chem.ubc.ca/muonic-helium-atom-new-scientist-2011 |department=Department of Chemistry. News & Events. |publisher=The University of British Columbia |access-date=11 February 2021 |quote=The chemical interactions of <sup>4</sup>H and <sup>4</sup>He-μ are then virtually identical.))</ref>((Better source needed|date=February 2021)) The muon could react with the protons in the nucleus, but in this case the reaction is not energetically favourable. Therefore, the atom decays with the muon's half-life, 1.52 microseconds (1.528312(±0.000015)×10<sup>−6</sup> seconds).((Cn|date=February 2021)) Other muonic atoms can be formed when negative muons interact with ordinary matter.<ref>((cite book | last1=Devons | first1=S. | last2=Duerdoth | first2=I. | year=1969 |chapter=Muonic Atoms | title=Advances in Nuclear Physics |editor1=Baranger, M. |editor2=Vogt, E. |publisher=Springer |pages=295–423 | doi=10.1007/978-1-4684-8343-7_5 | isbn=978-1-4684-8345-1))</ref>

== Hadronic atoms ==
A ''hadronic atom'' is an atom in which one or more of the [[atomic orbital|orbital electrons]] are replaced by a negatively charged [[hadron]].<ref>((cite book |title=Fundamentals in Hadronic Atom Theory |page=3 |first=A. |last=Deloff |location=River Edge, New Jersey |publisher=World Scientific |year=2003 |isbn=981-238-371-9))</ref> Possible hadrons include mesons such as the [[pion]] or [[kaon]], yielding a ''pionic atom''<ref>((cite journal |last1=Hori |first1=M. |last2=Aghai-Khozani |first2=H. |last3=Sótér |first3=A. |last4=Dax |first4=A. |last5=Barna |first5=D. |title=Laser spectroscopy of pionic helium atoms |journal=Nature |date=6 May 2020 |volume=581 |issue=7806 |pages=37–41 |doi=10.1038/s41586-020-2240-x |pmid=32376962 |bibcode=2020Natur.581...37H |s2cid=218527999))</ref> or a ''kaonic atom'' (see [[Kaonic hydrogen]]), collectively called ''mesonic atoms''; [[antiproton]]s, yielding an ''antiprotonic atom''; and the ((Subatomic particle|link=yes|Sigma-)) particle, yielding a ((Subatomic particle|Sigma-)) or ''sigmaonic atom''.<ref>p. 8, §16.4, §16.5, Deloff.</ref><ref name="ns">[https://www.newscientist.com/article/mg12717284-600/ The strange world of the exotic atom], Roger Barrett, Daphne Jackson and Habatwa Mweene, ''New Scientist'', August 4, 1990. accessdate=September 26, 2007.</ref><ref>p. 180, ''Quantum Mechanics'', B. K. Agarwal and Hari Prakash, New Delhi: Prentice-Hall of India Private Ltd., 1997. ((ISBN|81-203-1007-1)).</ref>

Unlike leptons, hadrons can interact via the [[strong force]], so the orbitals of hadronic atoms are influenced by [[nuclear force]]s between the [[atomic nucleus|nucleus]] and the hadron. Since the strong force is a short-range interaction, these effects are strongest if the atomic orbital involved is close to the nucleus, when the energy levels involved may broaden or disappear because of the absorption of the hadron by the nucleus.<ref name="as"/><ref name="ns"/> Hadronic atoms, such as pionic hydrogen and [[kaonic hydrogen]], thus provide experimental probes of the theory of strong interactions, [[quantum chromodynamics]].<ref>[https://cerncourier.com/a/exotic-atoms-cast-light-on-fundamental-questions/ Exotic atoms cast light on fundamental questions], ''CERN Courier'', November 1, 2006. accessdate=September 26, 2007.</ref>

== Onium ==
((Main|Onium))

An ''onium'' (plural: ''onia'') is the bound state of a particle and its antiparticle. The classic onium is [[positronium]], which consists of an electron and a positron bound together as a [[metastable]] state, with a relatively long lifetime of 142 ns in the triplet state.<ref name=adk>((cite journal|last1=Adkins|first1=G. S.|last2=Fell|first2=R. N.|last3=Sapirstein|first3=J.|title=Order α<sup>2</sup> Corrections to the Decay Rate of Orthopositronium|journal=Physical Review Letters|date=29 May 2000|volume=84|issue=22|pages=5086–5089|doi=10.1103/PhysRevLett.84.5086|pmid=10990873|arxiv= hep-ph/0003028 |bibcode= 2000PhRvL..84.5086A|s2cid=1165868))</ref> Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development called [[non-relativistic quantum electrodynamics]] (NRQED) used this system as a proving ground.

[[Pionium]], a bound state of two oppositely-charged [[pion]]s, is useful for exploring the [[strong interaction]]. This should also be true of [[protonium]], which is a proton–antiproton bound state. Understanding bound states of pionium and protonium is important in order to clarify notions related to [[exotic hadron]]s such as [[mesonic molecules]] and [[pentaquark]] states. [[Kaonium]], which is a bound state of two oppositely charged kaons, has not been observed experimentally yet.

The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain [[meson]]s, the ''[[quarkonium]] states'', which are made of a heavy quark such as the [[charm quark|charm]] or [[bottom quark]] and its antiquark. ([[Top quark]]s are so heavy that they decay through the [[weak force]] before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and [[lattice QCD]] are increasingly important tests of [[quantum chromodynamics]].

[[Muonium]], despite its name, is ''not'' an onium containing a muon and an antimuon, because IUPAC assigned that name to the system of an antimuon bound with an electron. However, the production of a muon–antimuon bound state, which ''is'' an onium (called [[true muonium]]), has been theorized.<ref>((cite web|url=https://www.sciencedaily.com/releases/2009/05/090529112609.htm|title=Theorists Reveal Path To True Muonium – Never-seen Atom|author=DOE/SLAC National Accelerator Laboratory|website=ScienceDaily|date=June 4, 2009|access-date=June 7, 2009))</ref>

== Hypernuclear atoms ==
((Main|Hypernucleus))

Atoms may be composed of electrons orbiting a [[hypernucleus]] that includes [[strange quark|strange]] particles called [[hyperon]]s. Such [[hypernucleus|hypernuclear atoms]] are generally studied for their nuclear behaviour, falling into the realm of [[nuclear physics]] rather than [[atomic physics]].

== Quasiparticle atoms ==
In [[condensed matter]] systems, specifically in some [[semiconductor]]s, there are states called [[exciton]]s, which are bound states of an electron and an [[electron hole]].

== Exotic molecules ==
An exotic molecule contains one or more exotic atoms.
*[[Di-positronium]], two bound positronium atoms
*[[Positronium hydride]], a positronium atom bound to a hydrogen atom

"Exotic molecule" can also refer to a molecule having some other uncommon property such as a pyramidal [[hexamethylbenzene#Dication]] and a [[Rydberg atom]].

== See also ==
((columns-list|colwidth=20em|
*[[Antihydrogen]]
*[[Antiprotonic helium]]
*[[Borromean nucleus]]
*[[Exotic matter]]
*[[Halo nucleus]]
*[[Kaonic hydrogen]]
*[[Lattice QCD]]
*[[Muonium]]
*[[Neutronium]]
*[[Pionium]]
*[[Positronium]]
*[[Quantum chromodynamics]]
*[[Quantum electrodynamics]]
*[[Quarkonium]]
))

== References ==
((Reflist))

((Particles))
((Authority control))

[[Category:Exotic atoms| ]]
[[Category:Quantum chromodynamics]]

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