The Hunting of the Quark

THE EXPERIMENTAL STATUS of quarks, hypothetical particles smaller than the electron, has excited debate within the physics community from 1977 on. The experimental developments that led up to this debate and the course the debate took up to mid-1979 reveal certain features of the experimental method that have received less attention from historians and philosophers than they deserve. In particular the quark debate reveals an intimate relationship between experimental practice and theoretical belief that has often been overlooked but has significant implications for our understanding of science. First, a thumbnail sketch of the history of quarks. In the early 1960s, with the advent of a new generation of particle accelerators, the list of elementary particles-strictly speaking the strongly interacting particles such as protons and neutrons, collectively known as "hadrons"-was growing rapidly. More than two hundred distinct hadrons have now been identified. In 1961 Murray Gell-Mann and Yuval Ne'eman introduced a symmetry scheme, known as "the eightfold way" or "SU(3)," which injected order into the proliferation of new hadrons by grouping them into multiplets, the members of each multiplet having closely related properties. I The success of this scheme has been unchallenged ever since. In 1964 Gell-Mann and George Zweig independently pointed out that this multiplet structure could be understood if one postulated the existence of a new layer of matter.2 They suggested that hadrons were not truly elementary particles, but were instead composites of just three subhadronic entities, which Gell-Mann christened "quarks." Combinations of quarks were supposed to make up hadrons, just as combinations of hadrons make up nuclei, combinations of nuclei and electrons make up atoms, and so on. The economy of Gell-Mann and Zweig's proposal was striking, explaining an apparently indefinitely rising number of hadrons in terms of only three constituent quarks. One question that immediately arose was, and is, could isolated quarks be found? Experimentalists began to search for them in a variety of ways, all predicated upon the unusual electric charges which had been postulated for quarks: either one third or two thirds of the electric charge of the electron (e). Since physicists had hitherto believed that all matter carried electric charge in