Editing J/psi meson (section)

== Background to discovery ==
The background to the discovery of the {{subatomic particle|J/Psi}} was both theoretical and experimental. In the 1960s, the first [[quark]] models of [[elementary particle physics]] were proposed, which said that [[proton]]s, [[neutron]]s, and all other [[baryon]]s, and also all [[meson]]s, are made from [[Fraction (mathematics)|fractionally]] charged particles, the "quarks", originally with three types or "flavors", called [[up quark|''up'']], [[down quark|''down'']],  and [[strange quark|''strange'']]. (Later the model was expanded to six quarks, adding the [[charm quark|''charm'']], [[top quark|''top'']] and [[bottom quark|''bottom'']] quarks.)  Despite the ability of quark models to bring order to the "elementary particle zoo", they were considered something like mathematical fiction at the time, a simple artifact of deeper physical reasons.<ref>
{{cite book
 |author=Pickering, A. 
 |year=1984
 |title=Constructing Quarks
 |pages=114–125
 |publisher=[[University of Chicago Press]]
 |isbn=978-0-226-66799-7
}}
</ref>

Starting in 1969, [[deep inelastic scattering]] experiments at [[SLAC]] revealed surprising experimental evidence for particles inside of protons. Whether these were quarks or something else was not known at first. Many experiments were needed to fully identify the properties of the sub-protonic components. To a first approximation, they indeed were a match for the previously described quarks.

On the theoretical front, [[gauge theory|gauge theories]] with [[broken symmetry]] became the first fully viable contenders for explaining the [[weak interaction]] after [[Gerardus 't&nbsp;Hooft]] discovered in 1971 how to calculate with them beyond [[Feynman diagram|tree level]]. The first experimental evidence for these [[electroweak force|electroweak unification]] theories was the discovery of the [[W and Z bosons|weak neutral current]] in 1973. Gauge theories with quarks became a viable contender for the [[strong interaction]] in 1973, when the concept of [[asymptotic freedom]] was identified.

However, a naive mixture of electroweak theory and the quark model led to calculations about known decay modes that contradicted observation: In particular, it predicted [[Z boson]]-mediated [[Flavor changing neutral current|''flavor-changing'']] decays of a strange quark into a down quark, which were not observed. A 1970 idea of [[Sheldon Glashow]], [[John Iliopoulos]], and [[Luciano Maiani]], known as the [[GIM mechanism]], showed that the flavor-changing decays would be strongly suppressed if there were a fourth quark (now called the ''[[charm quark]]'') that was a complementary counterpart to the [[strange quark]]. By summer 1974 this work had led to theoretical predictions of what a charm + anticharm meson would be like.

The group at [[Brookhaven National Laboratory|Brookhaven]],{{efn|name=Brookhaven-group|
Glenn Everhart, Terry Rhoades, Min Chen, and Ulrich Becker, at [[Brookhaven National Laboratory|Brookhaven]] first to discerned the 3.1&nbsp;GeV peak in  pair-production rates.}} were the first to discern a peak at 3.1&nbsp;GeV in plots of production rates. Ting named it the "J meson".<ref name="TingJ">''We discussed the name of the new particle for some time. Someone pointed out to me that the really exciting stable particles are designated by Roman characters – like the postulated W<sub>0</sub>, the intermediate vector boson, the Z<sub>0</sub>, etc. – whereas the "classical" particles have Greek designations like ρ, ω etc. This, combined with the fact that our work in the last decade had been concentrated on the electromagnetic current <math display="inline">j_\mu (x)</math> gave us the idea to call this particle the J particle.'' Samuel Ting, ''The Discovery of the J Particle'' Nobel prize lecture, 11. December 1976 [https://www.nobelprize.org/uploads/2018/06/ting-lecture.pdf]</ref>