Editing Exotic atom (section)

== 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>{{Cite web |title=Richard Feynman - Science Videos |url=http://vega.org.uk/video/subseries/8 |website=The Vega Science Trust}}</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.

Other muonic atoms can be formed when negative muons interact with ordinary matter.<ref name="Devons">{{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> The muon in muonic atoms can either decay or get captured by a proton. Muon capture is very important in heavier muonic atoms, but shortens the muon's lifetime from 2.2&nbsp;μs to only 0.08&nbsp;μs.<ref name="Devons"/>

=== 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 hydrogen atoms can form muonic hydrogen molecules. The spacing between the nuclei in such a molecule is hundreds of times smaller than in a normal hydrogen molecule – so close that the nuclei can spontaneously fuse together. This is known as ''[[muon-catalyzed fusion]]'', and was first observed between hydrogen-1 and deuterium nuclei in 1957.<ref name="Alvarez">{{cite journal | last1= Alvarez |first1=L.W. | year = 1957 | title = Catalysis of Nuclear Reactions by μ Mesons | journal = Physical Review | volume = 105 | issue = 3| page = 1127 |doi= 10.1103/PhysRev.105.1127 |bibcode = 1957PhRv..105.1127A | display-authors = 1 | last2 = Bradner | first2 = H. | last3 = Crawford | first3 = F. | last4 = Crawford | first4 = J. | last5 = Falk-Vairant | first5 = P. | last6 = Good | first6 = M. | last7 = Gow | first7 = J. | last8 = Rosenfeld | first8 = A. | last9 = Solmitz | first9 = F. |s2cid=123886206 }}</ref> Muon-catalyzed fusion has been proposed as a means of generating energy using fusion reactions in a room-temperature reactor.

=== 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 name=“Fleming”>
{{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 behaves more like a hydrogen atom than an inert helium atom. <ref name=“Fleming”/><ref>
{{Cite journal
 | title = Muonic alchemy: Transmuting elements with the inclusion of negative muons
 | journal = Chemical Physics Letters
 | volume = 539
 | pages = 209–221
 | doi = 10.1016/j.cplett.2012.04.062
 | last1 = Moncada | first1 = F.
 | last2 = Cruz | first2 = D.
 | last3 = Reyes | first3 = A
 |bibcode = 2012CPL...539..209M | year = 2012
}}</ref><ref>
{{Cite journal
 | title = Electronic properties of atoms and molecules containing one and two negative muons
 | journal = Chemical Physics Letters
 | volume = 570 | pages = 16–21
 | date =10 May 2013
 | doi = 10.1016/j.cplett.2013.03.004
 | last1 = Moncada | first1 = F.
 | last2 = Cruz | first2 = D.
 | last3 = Reyes | first3 = A.
 |bibcode = 2013CPL...570...16M
}}</ref>