Editing Muon (section)

=== Negative muon atoms ===
''Negative'' muons can form [[exotic atom|muonic atom]]s (previously called mu-mesic atoms), by replacing an electron in ordinary atoms. Muonic hydrogen atoms are much smaller than typical hydrogen atoms because the much larger mass of the muon gives it a much more localized [[ground state|ground-state]] [[wavefunction]] than is observed for the electron. In multi-electron atoms, when only one of the electrons is replaced by a muon, the size of the atom continues to be determined by the other electrons, and the atomic size is nearly unchanged. Nonetheless, in such cases, the orbital of the muon continues to be smaller and far closer to the nucleus than the [[atomic orbital]]s of the electrons.

Spectroscopic measurements in [[muonic hydrogen]] have been used to produce a precise estimate of the [[proton radius]].<ref name="Chargeradius">{{cite journal | last1 = Antognini | first1 = A. | last2 = Nez | first2 = F. | last3 = Schuhmann | first3 = K. | last4 = Amaro | first4 = F. D. | last5 = Biraben | first5 = F. | last6 = Cardoso | first6 = J. M. R. | last7 = Covita | first7 = D. S. | last8 = Dax | first8 = A. | last9 = Dhawan | first9 = S. | last10 = Diepold | first10 = M. | last11 = Fernandes | first11 = L. M. P. | last12 = Giesen | first12 = A. | last13 = Gouvea | first13 = A. L. | last14 = Graf | first14 = T. | last15 = Hänsch | first15 = T. W. | last16 = Indelicato | first16 = P. | last17 = Julien | first17 = L. | last18 = Kao | first18 = C. -Y. | last19 = Knowles | first19 = P. | last20 = Kottmann | first20 = F. | last21 = Le Bigot | first21 = E. -O. | last22 = Liu | first22 = Y. -W. | last23 = Lopes | first23 = J. A. M. | last24 = Ludhova | first24 = L. | last25 = Monteiro | first25 = C. M. B. | last26 = Mulhauser | first26 = F. | last27 = Nebel | first27 = T. | last28 = Rabinowitz | first28 = P. | last29 = Dos Santos | first29 = J. M. F. | last30 = Schaller | first30 = L. A. |display-authors=6 | title = Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen | journal = Science | volume = 339 | issue = 6118 | pages = 417–420 | year = 2013 | doi = 10.1126/science.1230016 | pmid = 23349284 |bibcode = 2013Sci...339..417A | hdl = 10316/79993 | s2cid = 346658 | url = http://doc.rero.ch/record/31960/files/kno_psm.pdf | hdl-access = free }}</ref> The results of these measurements diverged from the then accepted value giving rise to the so called [[proton radius puzzle]]. Later this puzzle found its resolution when new improved measurements of the proton radius in the electronic hydrogen became available.<ref name="KarrMarchand2019">{{cite journal |last1=Karr |first1=Jean-Philippe |last2=Marchand |first2=Dominique |title=Progress on the proton-radius puzzle |journal=Nature |volume=575 |issue=7781 |year=2019 |pages=61–62 |issn=0028-0836 |doi=10.1038/d41586-019-03364-z|pmid=31695215 |bibcode=2019Natur.575...61K |s2cid=207912706 |doi-access= }}</ref> 

Muonic [[helium]] is created by substituting a muon for one of the electrons in helium-4. The muon orbits much closer to the nucleus, so muonic helium can therefore be regarded like an isotope of helium whose nucleus consists of two neutrons, two protons and a muon, with a single electron outside. Chemically, muonic helium, possessing an unpaired [[valence electron]], can [[chemical bond|bond]] with other atoms, and behaves more like a hydrogen atom than an inert helium atom.<ref>
{{cite journal
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{{cite journal
 | title = Muonic alchemy: Transmuting elements with the inclusion of negative muons
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{{cite journal
 | title = Electronic properties of atoms and molecules containing one and two negative muons
 | journal = Chemical Physics Letters
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 | date =10 May 2013
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 | last1 = Moncada | first1 = F.
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Muonic heavy hydrogen atoms with a negative muon may undergo [[nuclear fusion]] in the process of [[muon-catalyzed fusion]], after the muon may leave the new atom to induce fusion in another hydrogen molecule. This process continues until the negative muon is captured by a helium nucleus, where it remains until it decays.

Negative muons bound to conventional atoms can be captured ([[muon capture]]) through the [[weak interaction|weak force]] by protons in nuclei, in a sort of electron-capture-like process. When this happens, [[nuclear transmutation]] results: The proton becomes a neutron and a muon neutrino is emitted.