THE 1990 NOBEL PRIZE FOR PHYSICS goes to Henry W. Kendall and Jerome I. Friedman of MIT, and Richard Taylor of Stanford for their study of high-energy electron-proton collisions conducted at the Stanford Linear Accelerator Center (SLAC) in the late 1960's. Their experiment demonstrated that quarks, the then supposed constituents of protons and neutrons, were not just mathematical constructs but real particles in their own right. The search for the elementary components of matter, a search dating back to antiquity, has in this century produced several significant results. In 1911 Ernest Rutherford showed that atoms consisted of a heavy nucleus and orbiting electrons. The nucleus in turn was found to be made up of neutrons and protons. In the 1950's and 1960's the discovery of numerous new particles, many of them living only fleetingly during high-energy collisions at accelerators, caused theorists to wonder if there weren't some additional underlying level of structure. In the quark model, formulated in the early 1960's by Murray Gell-Mann and others, all strongly interacting particles (hadrons) are thought to be composites made from even more elementary entities, the quarks. For example, baryons such as protons and neutrons are bound states consisting of three quarks while mesons are quark-antiquark states. But were the quarks anything more than a theoretical hypothesis?
In the MIT-Stanford experiments a large team of scientists, led by Kendall, Friedman, and Taylor, studied deep-inelastic interactions, in which beams of electrons were smashed into a stationary target of protons in the form of a vessel of liquid hydrogen or deuterium. Because the electrons do not feel the strong nuclear force, they are able to burrow deep into the protons and occasionally shatter them apart, creating a spray of debris particles. The analysis of these inelastic reactions indicated that when the electrons scattered from the protons, they were actually scattering from some smaller pointlike objects within the proton. (Firing rifle bullets at a bale of hay, you would soon discover whether or not an anvil were buried within the bale.) The pointlike objects inside the proton were quickly identified as quarks, providing a major empirical support for the quark model and laying the groundwork for what has come to be known as the "standard model" of elementary particle physics.
Articles: J.I. Friedman and H.W. Kendall, Annual Review of Nuclear Science 22, 203 (1972); S. Stein et al., Physical Review D12, 1884 (1975); Michael Riordan, "The Hunting of the Quark," Touchstone, 1987 (winner of the AIP Science Writing Award). Contacts: Kendall (617-253-7584), Friedman (617-253-7585), Taylor (415-926-2417), Stanford theorist Sidney Drell (415-926-3995), Riordan (415-926-2782).
