Published: 01 March 2014

In 1964, two physicists independently proposed the existence of the subatomic particles known as quarks.

Physicists Murray Gell-Mann and George Zweig were working independently on a theory for strong interaction symmetry in particle physics. Within this framework, they proposed that important properties of the strongly interacting particles – hadrons – could be explained if they were made up of constituent particles.

In 1961 Gell-Mann had introduced a symmetry scheme he called the Eightfold Way, which was based on the mathematical symmetry known as SU(3). The scheme (for which he received the Nobel prize in physics in 1969) classified the hadrons into two main groups, rather as the Periodic Table classifies the chemical elements.

Murray Gell-Mann, pictured here in the mid-1990s, authored a paper published on February 1, 1964, that described subatomic particles he called quarks.

Gell-Mann built upon this work in a new model that could successfully describe – among other things – the magnetic properties of protons and neutrons. But Gell-Mann's model required the existence of three new elementary particles, which he called "quarks".

By the end of 1963, Gell-Mann has written a short paper on the idea and sent it off to Physics Letters, where it was received on January 4, 1964 and published February 1. Titled “A Schematic Model of Baryons and Mesons,” it explained how various combinations of three particles from a triplet could produce baryons (such as protons and neutrons), while two members from the triplet could combine to form a meson (the most famous example at the time being the pi meson, or pion). Maintaining standard electric charges required a fourth particle for this approach to work, Gell-Mann noted in his paper. But “a simpler and more elegant scheme can be constructed if we allow non-integral values for the charges,” he wrote. “We then refer to the members … of the triplet as ‘quarks.’”

Gell-Mann says that he first came up with the sound "quork", and later chanced upon the phrase "Three quarks for Muster Mark" in James Joyce's Finnegans Wake. As Joyce presumably intended the word to rhyme with "Mark", people have been divided on the pronunciation ever since.

At about the same time, physicist George Zweig, had been working on similar ideas. He called his particles “aces”. George Zweig made his contribution to the field while he was a visitor to CERN in a paper dated 17 January 1964, in which he proposed: "Both mesons and baryons are constructed from a set of three fundamental particles called aces." After receiving his PhD from Caltech in 1963, Zweig spent 1963–1964 at CERN, where he wrote a paper entitled "An SU3 Model for Strong Interaction Symmetry and Its Breaking" (dated 17 January 1964) and a sequel (dated 21 February 1964). However, CERN required that papers written by junior members of the theory group be approved before they could be submitted for publication. Moreover, Zweig planned to publish in Physical Review, but CERN required that papers had to be published in European journals. Therefore approval was not forthcoming for these papers, so it took a while for his work to be fully appreciated. In 1964, Zweig returned to Caltech, where he was a professor in the high-energy theory group for 20 years. Unfortunately, Zweig was unable to attend the symposium.

Last September George Zweig visited CERN to give a colloquium entitled "Concrete Quarks; the Beginning of the End". In his talk, Zweig discussed the steps that led him to introduce his "aces" model while he was working in CERN’s Theory Division back in 1964, and also reviewed some of the main developments since that time. Following his work in the field of high-energy particle physics, Zweig moved to a totally different field, neurobiology, first doing experiments to understand how sound is represented in the auditory cortex of the cat and then moving on to the problem of characterizing cochlear mechanics.

George Zweig visited CERN and the ALICE cavern in September last year

Both Gell-Mann’s quarks and Zweig’s aces had to have electrical charges equal to 1/3 or 2/3 that of an electron or proton, suggesting that an experimental search for these constituents would reveal whether or not they existed.

In 1968, a series of electron-proton scattering experiments by the MIT-SLAC collaboration at the Stanford Linear Accelerator Center (SLAC) in the US revealed the first signs that nucleons have an inner structure. The team fired electrons at protons and observed how the electrons bounced off. The scattering patterns were identified as being caused by point-like particles inside the protons. In the subsequent years, by combining these results with others from neutrino-scattering in the Gargamelle bubble chamber at CERN, it became clear that these constituents really do have charges of 1/3 and 2/3.

In the decades that followed, new particles turned up that could not be built from the up, down and strange quarks, so the family was expanded. Today the quark chart lists three pairs: the up and down, strange and charm, and bottom and top. A fourth pair would presumably be called high and low, but most evidence suggests that no additional sets of quarks can exist. Quarks are now a key part of the Standard Model. ALICE and numerous other experiments at CERN study the properties of Gell-Mann and Zweig's particles with ever-greater precision.