The discovery of quarks

In the period following World War II, there was a rapid development of part icle 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 pl ayed an especially important role in uncovering the rich array of hadrons t hat 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 int o 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 h adrons, such as the Omega (-), were predicted within this framework and wer e 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) classifica tion scheme. When the quark model was first proposed, it postulated three t ypes 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 spin 1/2 part icle. 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 zero strangeness, they are (u,u,d) and (d,d,u) systems respectively. Though the quark model provided the best available tool for understanding t he properties of the hadrons that had been discovered at the time, the mode l was thought by many to be merely a mathematical representation of some de eper dynamics, but one of heuristic value. Among the reasons for this asses sment were the following: free quarks had not been found though they had be en sought in numerous accelerator and cosmic ray investigations and in sear ches in the terrestrial environment; there was a deep suspicion about the v alidity of their fractional charge assignments; and some of the baryon stat es constructed on the basis of the quark model violated the Pauli exclusion principle. Despite these difficulties there were a small number of theoris ts who continued to apply the model to explain a wide range of hadronic phe nomena. The theory of hadron structure that was most widely accepted at the time wa s the bootstrap model, an approach based on S-Matrix theory. This model, so metimes referred to as "nuclear democracy", was based on the idea that ther e were no fundamental particles and that all hadrons are made up of one ano ther. This picture was consistent with the low momentum transfer scattering seen in hadron-hadron interactions and with the observed "soft" electromag netic form factors of the proton and neutron; however, it could not provide the comprehensive description of multiplet structures that was given by th e quark model. Inelastic electron-nucleon scattering results, and later those from neutrin o-scattering, played a pivotal role in resolving this dilemma by firmly est ablishing the quark model. These experiments demonstrated that the proton a nd neutron are composite structures made up of point-like spin 1/2 constitu ents, with fractional charges consistent with those of quarks. More detailed descriptions of the deep inelastic program and its results ar e given in the written versions of the 1990 Nobel Lectures in Physics of Ri chard Taylor [5], Henry Kendall [6], and the author [7].