THE ELUSIVE QUARK
Ever since the modern era of atomic theory began in the 18th century, researchers have been unearthing clues about tinier and tinier constituents of the atom.
By the 1960s, technology such as high-energy particle accelerators and bubble chambers, which enabled scientists to observe particle collisions in great detail, helped unearth dozens of new particles.
Nobel laureate Murray Gell-Mann's "eightfold way" theory, also proposed by Yuval Ne'eman, brought order to the chaos created by the discovery of some 100 particles. Then he and (independently) George Zweig found that this successful classification scheme implied that all of those particles, including the neutron and proton, are composed of fundamental building blocks that Gell-Mann named "quarks."
The hunt was on. Scientists searched for them at acclerators, looked in seawater, air, cosmic rays and in the Earth. Not a quark was found. Even Gell-Mann admitted it was difficult to believe in quarks, and wrote that even if quarks were not real, they were still a useful mathematical tool.
Professor Friedman, the late Professor Henry Kendall of MIT, Professor Richard Taylor of the Stanford Linear Accelerator Center (SLAC) and a team of researchers from MIT and SLAC performed a series of electron-scattering experiments over seven years that provided the first direct evidence that there are point-like objects inside the proton.
But these objects were smaller than could be measured, and still there was disagreement. Sure, there were things inside protons, but were they quarks?
The comparison of electrons and neutrinos scattering off these point-like particles unequivocally demonstrated that these particles have the fractional charges assigned to quarks. The physics community was finally forced by inescapable and compelling evidence to accept quarks.
There are six kinds of quarks: up, down, strange, charm, bottom and top. All decay into up and down quarks, and these comprise matter as we know it.
