3. Nucleons and Quarks

Quark “flavours”

Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, Murray Gell-Mann of the United States and Yuval Neʾeman of Israel, proposed a particle classification scheme called the Eightfold Way, based on the mathematical symmetry group SU(3), which described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, having adopted the fanciful term from a passage in James Joyce’s novel Finnegans Wake. (The American physicist George Zweig developed a similar theory independently that same year and called his fundamental particles “aces.”) Gell-Mann’s model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It postulated the existence of three types of quarks, distinguished by unique “flavours.” These three quark types are now commonly designated as “up” (u), “down” (d), and “strange” (s). Each carries a fractional value of the electron charge (i.e., a charge less than that of the electron, e). The up quark (charge 2/3e) and down quark (charge −1/3e) make up protons and neutrons and are thus the ones observed in ordinary matter. Strange quarks (charge −1/3e) occur as components of K mesons and various other extremely short-lived subatomic particles that were first observed in cosmic rays but that play no part in ordinary matter.

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