Editing Pion (section)

== History ==
[[File:Nuclear Force anim smaller.gif|thumb|upright=1.6|An animation of the [[nuclear force]] (or residual strong force) interaction. The small colored double disks are gluons. For the choice of anticolors, see {{section link|Color charge|Red, green, and blue}}.]]
[[File:Pn Scatter Quarks.svg|thumb|upright=1.4|[[Feynman diagram]] for the same process as in the animation, with the individual [[quark]] constituents shown, to illustrate how the ''fundamental'' [[strong interaction]] gives rise to the nuclear force. Straight lines are quarks, while multi-colored loops are [[gluon]]s (the carriers of the fundamental force). Other gluons, which bind together the proton, neutron, and pion "in-flight", are not shown.<br/>The {{math|{{SubatomicParticle|Pion0}}}} meson contains an [[antiparticle|anti]]-quark, shown as travelling in the opposite direction, as per the [[Feynman–Stueckelberg interpretation]].]]
Theoretical work by [[Hideki Yukawa]] in 1935 had predicted the existence of [[meson]]s as the carrier particles of the [[strong nuclear force]]. From the range of the strong nuclear force (inferred from the radius of the [[atomic nucleus]]), Yukawa predicted the existence of a particle having a mass of about {{val|100|u=MeV/c2}}. Initially after its discovery in 1936, the [[muon]] (initially called the "mu meson") was thought to be this particle, since it has a mass of {{val|106|u=MeV/c2}}. However, later experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon a [[lepton]], and not a meson. However, some communities of astrophysicists continue to call the muon a "mu-meson".{{According to whom|date=January 2023}} The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950.

In 1947, the first true mesons, the charged pions, were found by the collaboration led by [[Cecil Powell]] at the [[University of Bristol]], in England. The discovery article had four authors: [[César Lattes]], [[Giuseppe Occhialini]], [[Hugh Muirhead]] and Powell.<ref>{{cite journal |author=C. Lattes, G. Occhialini, H. Muirhead and C. Powell |year=1947 |title=Processes Involving Charged Mesons |journal=[[Nature (journal)|Nature]] |volume=159 |issue=1 |pages=694–698 |doi=10.1007/s00016-014-0128-6|bibcode=2014PhP....16....3V |s2cid=122718292 }}</ref> Since the advent of [[particle accelerator]]s had not yet come, high-energy subatomic particles were only obtainable from atmospheric [[cosmic ray]]s. [[Photographic emulsion]]s based on the [[gelatin-silver process]] were placed for long periods of time in sites located at high-altitude mountains, first at [[Pic du Midi de Bigorre]] in the [[Pyrenees]], and later at [[Chacaltaya]] in the [[Andes Mountains]], where the plates were struck by cosmic rays.
After development, the [[photographic plate]]s were inspected under a [[microscope]] by a team of about a dozen women.<ref>{{cite journal |author=C. L. Vieria, A. A. P Videira |year=2014 |title=Cesar Lattes, Nuclear Emulsions, and the Discovery of the Pi-meson |journal=Physics in Perspective |volume=16 |issue=1 |pages=2–36 |doi=10.1007/s00016-014-0128-6|bibcode=2014PhP....16....3V |s2cid=122718292 }}</ref> [[Marietta Kurz]] was the first person to detect the unusual "double meson" tracks, characteristic for a pion decaying into a [[muon]], but they were too close to the edge of the photographic emulsion and deemed incomplete. A few days later, Irene Roberts observed the tracks left by pion decay that appeared in the discovery paper. Both women are credited in the figure captions in the article.

In 1948, [[César Lattes|Lattes]], [[Eugene Gardner]], and their team first artificially produced pions at the [[University of California at Berkeley|University of California]]'s [[cyclotron]] in [[Berkeley, California]], by bombarding [[carbon]] atoms with high-speed [[alpha particle]]s. Further advanced theoretical work was carried out by [[Riazuddin (physicist)|Riazuddin]], who in 1959 used the [[dispersion relation]] for [[Compton scattering]] of [[virtual photon]]s on pions to analyze their charge radius.<ref>{{cite journal |author=Riazuddin |year=1959 |title=Charge radius of the pion |journal=[[Physical Review]] |volume=114 |issue=4 |pages=1184–1186 |bibcode=1959PhRv..114.1184R |doi=10.1103/PhysRev.114.1184}}</ref>

Since the neutral pion is not [[electric charge|electrically charged]], it is more difficult to detect and observe than the charged pions are. Neutral pions do not leave tracks in photographic emulsions or Wilson [[cloud chamber]]s. The existence of the neutral pion was inferred from observing its decay products from [[cosmic ray]]s, a so-called "soft component" of slow electrons with photons. The {{math|{{SubatomicParticle|Pion0}}}} was identified definitively at the University of California's cyclotron in 1949 by observing its decay into two photons.<ref>{{cite journal |first1=R. |last1=Bjorklund |first2=W.E. |last2=Crandall |first3=B.J. |last3=Moyer |first4=H.F. |last4=York |year=1949 |title=High energy photons from proton–nucleon collisions |journal=[[Physical Review]] |volume=77 |issue=2 |pages=213–218 |bibcode=1950PhRv...77..213B |doi=10.1103/PhysRev.77.213 |hdl=2027/mdp.39015086480236 |url=https://escholarship.org/content/qt9qd089vt/qt9qd089vt.pdf?t=p0uv3w}}</ref> Later in the same year, they were also observed in cosmic-ray balloon experiments at Bristol University.

{{blockquote|... Yukawa choose the letter {{math|π}} because of its resemblance to the [[Kanji]] character for [[Wikt:介#Japanese|介]] [''kai''], which means "to mediate", based on the idea that the meson works as a strong force mediator particle between hadrons.<ref>{{cite AV media |last=Zee |first=Anthony |author-link=Anthony Zee |date=7 December 2013 |title=Quantum Field Theory, Anthony Zee {{!}} Lecture&nbsp;2 of 4 (lectures given in 2004) |publisher=aoflex |medium=video |via=YouTube |url=https://www.youtube.com/watch?v=aypGTsLN0ck }} (quote at 57{{sup|m}}04{{sup|s}} of 1{{sup|h}}26{{sup|m}}39{{sup|s}})</ref>}}