Editing Exotic atom (section)

== 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 state 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> The same applies to the [[Tau_(particle)#Exotic_atoms|ditauonium (or "true tauonium")]] exotic QED atom.<ref name="dEnterria1">
{{cite journal
 | last1=d'Enterria| first1=David
 | last2=Perez-Ramos| first2=Redamy
 | last3=Shao| first3=Hua-Sheng
 | year=2022
 | title=Ditauonium spectroscopy
 | journal=[[European Physical Journal C]]
 | volume=82  | issue=10
 | page=923
 | arxiv=2204.07269 | bibcode=  2022EPJC...82..923D| doi=10.1140/epjc/s10052-022-10831-x | pmid=
 | s2cid=248218441
}}</ref>