Editing Meson (section)

== History ==
From theoretical considerations, in 1934 [[Hideki Yukawa]]<ref>{{cite web |publisher=The Noble Foundation |year=1949 |title=Nobel Prize in Physics 1949 |series=Presentation Speech |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1949/press.html}}</ref><ref name="H. Yukawa, (1935)">{{cite journal |last=Yukawa |first=H. |year=1935 |title=On the Interaction of Elementary Particles |journal=Proc. Phys.-Math. Soc. Jpn. |volume=17 |issue=48  |url=http://web.ihep.su/dbserv/compas/src/yukawa35/eng.pdf}}</ref> predicted the existence and the approximate mass of the "meson" as the carrier of the [[nuclear force]] that holds [[atomic nucleus|atomic nuclei]] together.<ref>{{cite journal |first=Hideki |last=Yukawa |year=1935 |title=On the Interaction of Elementary Particles. I |journal=Nippon Sugaku-Buturigakkwai Kizi Dai 3 Ki |volume=17 |pages=48–57 |doi=10.11429/ppmsj1919.17.0_48 |publisher=日本物理学会、日本数学会 |url=https://www.jstage.jst.go.jp/article/ppmsj1919/17/0/17_0_48/_pdf/-char/en}}</ref> If there were no nuclear force, all nuclei with two or more [[proton]]s would fly apart due to [[Electromagnetism|electromagnetic]] repulsion. [[Hideki Yukawa|Yukawa]] called his carrier particle the meson, from μέσος ''mesos'', the [[Ancient Greek|Greek]] word for "intermediate", because its predicted mass was between that of the electron and that of the proton, which has about 1,836&nbsp;times the mass of the electron. [[Hideki Yukawa|Yukawa]] or [[Carl David Anderson]], who discovered the [[muon]], had originally named the particle the "mesotron", but he was corrected by the physicist [[Werner Heisenberg]] (whose father was a professor of Greek at the [[University of Munich]]). Heisenberg pointed out that there is no "tr" in the Greek word "mesos".<ref name="G. Gamow, (1961)">{{cite book |last1=Gamow |first1=G. |orig-year=1961 |title=The Great Physicists from Galileo to Einstein |publisher=Dover Publications |year=1988 |isbn=978-0-486-25767-9 |edition=Reprint |url=https://archive.org/details/greatphysicistsf0000gamo |url-access=registration |page=[https://archive.org/details/greatphysicistsf0000gamo/page/315 315] }}</ref>

The first candidate for Yukawa's meson, in modern terminology known as the [[muon]], was discovered in 1936 by [[Carl David Anderson]] and others in the [[decay product]]s of cosmic ray interactions. The [[muon|"mu meson"]] had about the right mass to be Yukawa's carrier of the strong nuclear force, but over the course of the next decade, it became evident that it was not the right particle. It was eventually found that the [[muon|"mu meson"]] did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the [[electron]], and was eventually classed as a [[lepton]] like the electron, rather than a meson. Physicists in making this choice decided that properties other than particle mass should control their classification.

There were years of delays in the subatomic particle research during [[World War II]] (1939–1945), with most physicists working in applied projects for wartime necessities. When the war ended in August&nbsp;1945, many physicists gradually returned to peacetime research. The first true meson to be discovered was what would later be called the [[pion|"pi meson"]] (or pion). During 1939–1942, [[Debendra Mohan Bose]] and [[Bibha Chowdhuri]] exposed [[Ilford Photo|Ilford]] [[half-tone]] photographic plates in the high altitude mountainous regions of [[Darjeeling]], and observed long curved ionizing tracks that appeared to be different from the tracks of alpha particles or protons. In a series of articles published in ''[[Nature (journal)|Nature]]'', they identified a cosmic particle having an average mass close to 200 times the mass of electron.<ref name="science_and_culture_76">
{{cite journal
 | title = D. M. Bose: A Scientist Incognito (editorial)
 | volume = 76
 | issue = 11–12
 | journal = Science and Culture
 | date=November–December 2010
 | url = http://www.scienceandculture-isna.org/Nov-Dec-10/Editorial.pdf
 | access-date = 5 February 2011
}}</ref> This discovery was made in 1947 with improved full-tone photographic emulsion plates, by [[Cecil Powell]], [[Hugh Muirhead]], [[César Lattes]], and [[Giuseppe Occhialini]], who were investigating cosmic ray products at the [[University of Bristol]] in [[England]], based on photographic films placed in the Andes mountains.<ref>{{cite journal |author1=Lattes, C. |author2=Occhialini, G. |author3=Muirhead, H. |author4=Powell, C. |year=1947 |title=Processes involving charged mesons |journal=[[Nature (journal)|Nature]] |volume=159 |pages=694–698 |doi=10.1007/s00016-014-0128-6|s2cid=122718292 }}</ref> Some of those mesons had about the same mass as the already-known mu "meson", yet seemed to decay into it, leading physicist [[Robert Marshak]] to hypothesize in 1947 that it was actually a new and different meson. Over the next few years, more experiments showed that the pion was indeed involved in strong interactions. The pion (as a [[virtual particle]]) is also used as force carrier to model the [[nuclear force]] in [[atomic nucleus|atomic nuclei]] (between [[proton|protons]] and [[neutron|neutrons]]). This is an approximation, as the actual carrier of the strong force is believed to be the [[gluon]], which is explicitly used to model strong interaction between quarks. Other mesons, such as the virtual [[rho meson]]s are used to model this force as well, but to a lesser extent. Following the discovery of the pion, Yukawa was awarded the 1949 [[Nobel Prize in Physics]] for his predictions.

For a while in the past, the word ''meson'' was sometimes used to mean ''any'' force carrier, such as [[W and Z bosons|"the Z{{sup|0}} meson"]], which is involved in mediating the [[weak interaction]].<ref name ="J. Steinberger, (1998)">{{cite journal |last1=Steinberger |first1=J. |year=1989 |title=Experiments with high-energy neutrino beams |url=https://cds.cern.ch/record/193654 |journal=[[Reviews of Modern Physics]] |volume=61 |issue=3 |pages=533–545 |bibcode = 1989RvMP...61..533S |doi=10.1103/RevModPhys.61.533 |pmid=17747881 }}</ref> However, this use has fallen out of favor, and mesons are now defined as particles composed of pairs of quarks and antiquarks.