Pais Prize Session

Why do particle physicists consider quarks and gluons to be real, physical corpuscles when they have never witnessed one of them leaving a track in their particle detectors? This is an important epistemological question that deserves more attention than it has thus far received in the historical and philosophical literature. It was a core concern of my 1987 book, The Hunting of the Quark.

            After a few introductory reflections about the origins of the Abraham Pais Prize and on the particle physicist and science historian who heavily influenced my research and writings, I will address the reality of quarks and gluons based partly on my research in high-energy physics. During the late 1960s and early 1970s, unexpected new features began turning up in a series of electron-nucleon scattering experiments at the Stanford Linear Accelerator Center, in which I took part as an MIT graduate student and postdoctoral researcher. By 1973 the best explanation of these data was that we were striking fractionally charged, spin-1/2 quarks at the heart of the proton and neutron. That same year, an asymptotically free gauge theory — soon recognized as "quantum chromodynamics," or QCD — provided a cogent explanation of why these quarks can never appear beyond the bounds of their parent hadrons. Quarks bearing a new quantum number dubbed "color" also helped account for other new phenomena, such as the strikingly narrow J/psi resonance that appeared in 1974 Brookhaven and SLAC experiments. And during the late 1970s, compact "jets" of hadrons were interpreted using QCD as the outwardly physical manifestations of quarks and gluons produced in energetic electron-positron collisions at SLAC and the German DESY laboratory. Along with the by-then overwhelming statistical evidence for their existence, it was this strikingly visual evidence that finally convinced the particle physics community that quarks and gluons were indeed "real."