Editing Muon (section)

=== Positive muon atoms ===
A ''positive'' muon, when stopped in ordinary matter, cannot be captured by a proton since the two positive charges can only repel. The positive muon is also not attracted to the nucleus of atoms. Instead, it binds a random electron and with this electron forms an exotic atom known as [[muonium]] (mu) atom. In this atom, the muon acts as the nucleus. The positive muon, in this context, can be considered a pseudo-isotope of hydrogen with one ninth of the mass of the proton. Because the mass of the electron is much smaller than the mass of both the proton and the muon, the [[reduced mass]] of muonium, and hence its [[Bohr radius]], is very close to that of [[hydrogen]]. Therefore this bound muon-electron pair can be treated to a first approximation as a short-lived "atom" that behaves chemically like the isotopes of hydrogen ([[Hydrogen-1|protium]], [[deuterium]] and [[tritium]]).

Both positive and negative muons can be part of a short-lived pi-mu atom consisting of a muon and an oppositely charged pion. These atoms were observed in the 1970s in experiments at [[Brookhaven National Laboratory]] and [[Fermilab]].<ref>{{cite journal |last1=Coombes |first1=R. |last2=Flexer |first2=R. |last3=Hall |first3=A. |last4=Kennelly |first4=R. |last5=Kirkby |first5=J. |last6=Piccioni |first6=R. |last7=Porat |first7=D. |last8=Schwartz |first8=M. |last9=Spitzer |first9=R. |last10=Toraskar |first10=J. |last11=Wiesner |first11=S. |last12=Budick |first12=B. |last13=Kast |first13=J. W. |display-authors=6 |title=Detection of π−μ coulomb bound states |journal=Physical Review Letters |publisher=American Physical Society (APS) |volume=37 |issue=5 |pages=249–252 |date=1976-08-02 |df=dmy-all |issn=0031-9007 |doi=10.1103/physrevlett.37.249}}</ref><ref>{{cite journal |last1=Aronson |first1=S. H. |last2=Bernstein |first2=R. H. |last3=Bock |first3=G. J. |last4=Cousins |first4=R. D. |last5=Greenhalgh |first5=J. F. |last6=Hedin |first6=D. |last7=Schwartz |first7=M. |last8=Shea |first8=T. K. |last9=Thomson |first9=G. B. |last10=Winstein |first10=B. |display-authors=6 |title=Measurement of the rate of formation of pi-mu atoms in <math>K_{L}^{0}</math> decay |journal=Physical Review Letters |publisher=American Physical Society (APS) |volume=48 |issue=16 |pages=1078–1081 |date=1982-04-19 |df=dmy-all |issn=0031-9007 |doi=10.1103/physrevlett.48.1078|bibcode=1982PhRvL..48.1078A }}</ref>