Focus: Landmarks—Discovery of Particles inside the Proton
Landmarks articles feature important papers from the archives of the Physical Review journals.
Two 1969 reports in Physical Review Letters described what happened when high-energy electrons smashed into hydrogen atoms. These experiments probed the structure of the proton and triggered an exchange between experimenters and theorists that ultimately solidified the idea that protons, neutrons, and related heavy particles are made of constituent particles called quarks. The experiments were recognized with the 1990 Nobel Prize in Physics.
The quark model arose in 1964, when physicists Murray Gell-Mann, then at the California Institute of Technology (Caltech) in Pasadena, and George Zweig, then at CERN, the European particle physics lab, independently proposed a classification system for the large number of particles in the so-called hadron family. The idea was that each hadron was made up of a different combination of quarks (as Gell-Mann called them), particles with the oddity of having fractional charge. However, collision experiments involving protons failed to shake any of these hypothetical particles loose, recalls Jerome Friedman of the Massachusetts Institute of Technology (MIT). Instead, the results were consistent with a proton that was a featureless blob of charge.
In the late 1960s, Friedman, MIT colleague Henry Kendall, and Stanford Linear Accelerator Center (SLAC) physicist Richard Taylor were part of a team that bombarded hydrogen targets using the new high-energy electron beam at SLAC. They first studied elastic collisions, in which the electron and proton bounce off each other like ideal billiard balls. The number of scattered electrons detected at a given angle (the scattering rate) decreased with increasing electron energy just as expected if the proton were simply a charged blob.
Although these results were “not very exciting,” as Friedman puts it, the team went on to study inelastic collisions, in which the proton absorbs some of the electron’s kinetic energy internally and blows apart. The experimenters found that for electrons with energies of to giga-electron-volts, the scattering rate was to times greater than the prediction for a featureless proton structure.
Around the same time, SLAC theorist James Bjorken made predictions of the electron-proton interaction using current algebra, a mathematical technique that Gell-Mann adapted in an early attempt to understand the behavior of quarks. Bjorken based his predictions on the two experimental parameters that characterize inelastic collisions: the electron kinetic energy absorbed internally by the proton and the momentum transferred from the electron to the proton. Bjorken found that the scattering rate should decrease relatively slowly for increasingly large electron energies, according to an expression (called a scaling law) that, surprisingly, depended only on a simple combination of the two parameters .
Remarkably, the scattering results fit nicely with Bjorken’s prediction, but “we didn’t understand what scaling implied,” Friedman says. Richard Feynman of Caltech then learned of the results. He realized that because of special relativity, an electron traveling close to the speed of light would see a stationary proton flattened into a pancake-shaped object. He then imagined that this proton disk contained some number of noninteracting constituent particles, which he called partons. From this model he was able to obtain the same scaling law that Bjorken had laboriously derived from current algebra .
In the first of the two 1969 papers by Friedman and his colleagues, the team described their experiments. In the second, they evaluated the data in the context of various theories of the time, including quarks, partons, and Bjorken’s and Feynman’s scaling results, along with several other ideas that soon lost favor. Further experiments of the same type—which became known as deep inelastic scattering experiments because they looked deep within hadrons—eventually confirmed that there were three particles inside the proton, with fractional charges. But definitive identification of the observed partons with the theoretical quarks had to await further work to resolve a contradiction: Quarks were evidently so tightly bound that they could not be pried out of hadrons, while the partons were assumed to rattle around freely. The 1973 theory of asymptotic freedom solved that problem and received the 2004 physics Nobel Prize [see Nobel Focus, 2004].
Friedman, who won the 1990 Nobel Prize in Physics with Kendall and Taylor, describes their story as one “in which theory and experiment intermix.” Bjorken says that the interpretation of his scaling law as evidence for constituent particles in the proton “was there as an option, but a chancy one. It did not really come into its own until post-Feynman.” Friedman agrees that Feynman turned confusion into clarity, but he calls Bjorken the “unsung hero” of the tale.
David Lindley is a freelance writer in Alexandria, Virginia, and author of Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science (Doubleday, 2007).
- J. D. Bjorken, “Asymptotic Sum Rules at Infinite Momentum,” Phys. Rev. 179, 1547 (1969)
- Feynman never published this result, but Bjorken and a colleague worked out the consequences of the idea: J. D. Bjorken and E. A. Paschos, “Inelastic Electron-Proton and γ-Proton Scattering and the Structure of the Nucleon,” Phys. Rev. 185, 1975 (1969)