Editing Muon (section)

== History of discovery ==
Muons were discovered by [[Carl D. Anderson]] and [[Seth Neddermeyer]] at [[California Institute of Technology|Caltech]] in 1936 while studying [[cosmic radiation]]. Anderson noticed particles that curved differently from electrons and other known particles when passed through a [[magnetic field]]. They were negatively charged but curved less sharply than electrons, but more sharply than [[proton]]s, for particles of the same velocity. It was assumed that the magnitude of their negative electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that their mass was greater than an electron's but smaller than a proton's. Thus Anderson initially called the new particle a ''mesotron'', adopting the prefix ''meso-'' from the Greek word for "mid-". The existence of the muon was confirmed in 1937 by [[J. Curry Street|J. C.&nbsp;Street]] and E. C.&nbsp;Stevenson's [[cloud chamber]] experiment.<ref>{{cite journal |doi=10.1103/PhysRev.52.1003 |title=New evidence for the existence of a particle of mass intermediate between the proton and electron |journal=Physical Review |volume=52 |issue=9 |page=1003 |year=1937 |last1=Street |first1=J. |last2=Stevenson |first2=E. |bibcode=1937PhRv...52.1003S |s2cid=1378839 }}</ref>

A particle with a mass in the [[meson]] range had been predicted before the discovery of any mesons, by theorist [[Hideki Yukawa]]:<ref>{{cite journal |author=Yukawa, Hideki |url=http://web.ihep.su/dbserv/compas/src/yukawa35/eng.pdf |title=On the interaction of elementary particles |journal=Proceedings of the Physico-Mathematical Society of Japan |year=1935 |volume=17 |issue=48 |pages=139–148}}</ref>

<blockquote>
It seems natural to modify the theory of Heisenberg and Fermi in the following way. The transition of a heavy particle from neutron state to proton state is not always accompanied by the emission of light particles. The transition is sometimes taken up by another heavy particle.
</blockquote>

Because of its mass, the mu meson was initially thought to be Yukawa's particle and some scientists, including [[Niels Bohr]], originally named it the ''yukon''. 
The fact that the mesotron (i.e. the muon) was not Yukawa's particle was established in 1946 by an experiment conducted by [[Marcello Conversi]], [[Oreste Piccioni]], and Ettore Pancini in Rome. In this experiment, which [[Luis Walter Alvarez]] called the "start of modern particle physics" in his 1968 Nobel lecture,<ref name=Nobel>{{cite journal |last1=Alvarez |first1=Luis W. |date=11 December 1968 |title=Recent developments in particle physics |url=http://www.nobelprize.org/nobel_prizes/physics/laureates/1968/alvarez-lecture.pdf |journal=Nobel Lecture |access-date=17 July 2017 }}</ref> they showed that the muons from cosmic rays were decaying without being captured by atomic nuclei, contrary to what was expected of the mediator of the [[nuclear force]] postulated by Yukawa.  Yukawa's predicted particle, the [[pi meson]], was finally identified in 1947 (again from cosmic ray interactions).

With two particles now known with the intermediate mass, the more general term ''meson'' was adopted to refer to any such particle within the correct mass range between electrons and nucleons. Further, in order to differentiate between the two different types of mesons after the second meson was discovered, the initial mesotron particle was renamed the ''mu meson'' (the Greek letter ''μ'' [''mu''] corresponds to ''m''), and the new 1947 meson (Yukawa's particle) was named the ''pi meson''.

As more types of mesons were discovered in accelerator experiments later, it was eventually found that the mu meson significantly differed not only from the pi meson (of about the same mass), but also from all other types of mesons. The difference, in part, was that mu mesons did not interact with the nuclear force, as pi mesons did (and were required to do, in Yukawa's theory). Newer mesons also showed evidence of behaving like the pi meson in nuclear interactions, but not like the mu meson. Also, the mu meson's decay products included both a [[neutrino]] and an [[antineutrino]], rather than just one or the other, as was observed in the decay of other charged mesons.

In the eventual [[Standard Model]] of [[particle physics]] codified in the 1970s, all mesons other than the mu meson were understood to be [[hadrons]] – that is, particles made of [[quarks]] – and thus subject to the nuclear force. In the quark model, a ''meson'' was no longer defined by mass (for some had been discovered that were very massive – more than [[nucleon]]s), but instead were particles composed of exactly two quarks (a quark and antiquark), unlike the [[baryon]]s, which are defined as particles composed of three quarks (protons and neutrons were the lightest baryons). Mu mesons, however, had shown themselves to be fundamental particles (leptons) like electrons, with no quark structure. Thus, mu "mesons" were not mesons at all, in the new sense and use of the term ''meson'' used with the quark model of particle structure.

With this change in definition, the term ''mu meson'' was abandoned, and replaced whenever possible with the modern term ''muon'', making the term "mu meson" only a historical footnote. In the new quark model, other types of mesons sometimes continued to be referred to in shorter terminology (e.g., ''pion'' for pi meson), but in the case of the muon, it retained the shorter name and was never again properly referred to by older "mu meson" terminology.

The eventual recognition of the muon as a simple "heavy electron", with no role at all in the nuclear interaction, seemed so incongruous and surprising at the time, that Nobel laureate [[Isidor Isaac Rabi|I.&nbsp;I.&nbsp;Rabi]] famously quipped, "Who ordered that?"<ref name=Bartusiak>{{cite news |url=https://www.nytimes.com/1987/09/27/books/science-technology-who-ordered-the-muon.html |department=Science & Technology |title=Who ordered the muon? |first=Marcia |last=Bartusiak |newspaper=[[The New York Times]] |date=27 September 1987 |access-date=30 August 2016}}</ref>

In the [[Experimental testing of time dilation#Rossi–Hall experiment|Rossi–Hall experiment]] (1941), muons were used to observe the [[time dilation]] (or, alternatively, [[length contraction]]) predicted by [[special relativity]], for the first time.<ref>{{cite journal | last = Self | first = Sydney | title = APPLICATION OF GENERAL SEMANTICS TO THE NATURE OF TIME HISTORY | date = 2018 | journal = Etc: A Review of General Semantics | volume = 75 | issue = 1–2 | pages = 162–166}}</ref>