The discovery of quarks

In the period following World War II, there was a rapid development of particle physics. With the construction of synchrotrons and the development of detector technology, many new particles were discovered and the systematics of their interactions investigated. The invention of the bubble chamber played an especially important role in uncovering the rich array of hadrons that were discovered in this period. In 1961 Murray Gell-Mann [1] and Yuval Ne'eman [2] independently introduced a classification scheme, based on SU(3) symmetry, which placed hadrons into families on the basis of spin and parity. Like the periodic table for the elements, this scheme was predictive as well as descriptive, and various hadrons, such as the Ω—, were predicted within this framework and were later discovered. In 1964 Gell-Mann [3] and George Zweig [4] independently proposed quarks as the building blocks of hadrons as a way of generating the SU(3) classification scheme. When the quark model was first proposed, it postulated three types of quarks: up (u), down (d), and strange (s), with charges 2/3, —1/3, and —1/3 respectively. Each of these was hypothesized to be a spin1/2 particle. In this model the nucleon (and all other baryons) is made up of three quarks, and each meson consists of a quark and an antiquark. For example, as the proton and neutron both have ero strangeness, they are (u,u,d) and (d,d,u) systems respectively. Though the quark model provided the best available tool for understanding the properties of the hadrons that had been discovered at the time, the model was thought by many to be merely a mathematical representation of some deeper dynamics, but one of heuristic value. Among the reasons for this assessment were the following:free quarks had not been found though they had been sought in numerous accelerator and cosmic ray investigations and in searches in the terrestrial environment; there was a deep suspicion about the validity of their fractional charge assignments; and some of the baryon states constructed on the basis of the quark model violated the Pauli exclusion principle. Despite these difficulties there were a small number of theorists who continued to apply the model to explain a wide range of hadronic phenomena. The theory of hadron structure that was most widely accepted at the time was the bootstrap model, an approach based on S-Matrix theory. This model, sometimes referred to as “nuclear democracy,” was based on the idea that there were no fundamental particles and that all hadrons are made up of one another. This picture was consistent with the low momentum transfer scattering seen in hadron-hadron interactions and with the observed “soft” electromagnetic form factors of the proton and neutron; however, it could not provide the comprehensive description of multiplet structures that was given by the quark model. Inelastic electron-nucleon scattering results, and later those from neutrino-scattering, played a pivotal role in resolving this dilemma by firmly establishing the quark model. These experiments demonstrated that the proton and neutron are composite structures made up of point-like spin 1/2 constituents, with fractional charges consistent with those of quarks. More detailed descriptions of the deep inelastic program and its results are given in the written versions of the 1990 Nobel Lectures in Physics of Richard Taylor [5], Henry Kendall [6], and the author [7].