Quantum Chromodynamics, or QCD, is the theory of strong interactions between quarks and gluons, which taken together with Weinberg-Salam Gauge Theory of the Electroweak Interactions (1979 Physics Nobel), constitute the very successful "Standard Model" of present day elementary particle physics. Feynman argued that high-energy experiments showed that quarks are real particles, which he called partons. Feynman's younger Caltech colleague, Murray Gell-Mann, who won the 1969 Nobel Prize, himself, had coined the term quark for these constituents. While Gell-Mann and Feynman did important work together at Caltech in 1957 regarding an early theory of the weak interactions (Gleick, pp. 330-9), their ideas began to diverge later in life, and their differences in approach regarding QCD reflected a deep, but temporary rift in the theoretical physics community.
The pair of Feynman diagrams, upper right, illustrate the nuclear scattering of a proton (P) and a neutron (N). In the old Yukawa theory of the WWII era, this strong interaction was mediated by a pion, here indicated by the pi+. Within the later context of QCD, this scattering event is understood at a deeper level— Here the proton (uud), consisting of two "up" quarks, each of charge +2/3, and a "down" quark of charge -1/3, interacts with a neutron (ddu), which being two downs & an up is electrically neutral, via the exchange of the positively charged pion, made of an up quark (u) & down antiquark (d bar).
In Feynman's list of quarks, 1974 and 1978 refer to the dates that the "charm" and "bottom/beauty" quarks were discovered at SLAC/BNL and Fermilab, respectively. The 6th quark, indicated here as "t" by Feynman refers to the "top", sometimes called "truth", and was not discovered until 1995, well after Feynman's passing, though had been predicted, along with the bottom quark, in 1973 by Kobayashi & Maskawa, for which they were awarded the 2008 Nobel.
The lower Weak interaction diagram shows the decay of a negatively charge muon (mu) into an electron (e), a muon neutrino and an electron anti-neutrino, the neutrinos (n), appropriately sub or superscripted, with an overbar indicating anti-matter, which travels backward in space-time.
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