Fifty years of quarks

January 20, 2014 by Cian O'luanaigh, Cern
Murray Gell-Mann visited CERN and the ATLAS experiment in January last year. Credit: Maximilien Brice/CERN

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.

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 "."

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.

Physicist 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 called aces." Though Zweig's name for the particles did not stick, he showed that some properties of hadrons could be explained by treating them as triplets of other constituent particles.

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.

George Zweig visited CERN and the ALICE cavern in September last year. Credit: Panagiotis Charitos

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.

Quarks are now a key part of the Standard Model. In numerous experiments at CERN including those at the Large Hadron Collider (LHC), physicists are measuring the properties of Gell-Mann and Zweig's with ever-greater precision.

Explore further: Hunt for the platypus particle

Related Stories

Hunt for the platypus particle

October 22, 2012

All of the atoms in our bodies are made of electrons, protons and neutrons, and the protons and neutrons can be further broken down into quarks. Fundamentally, then, we are made of only two types of particles: electrons and ...

12 matter particles suffice in nature

December 13, 2012

How many matter particles exist in nature? Particle physicists have been dealing with this question for a long time. The 12 matter particles contained in the standard model of particle physics? Or are there further particles ...

Physicists publish observation of the 'Charming Socialites'

December 20, 2013

Protons and neutrons, the particles in an atomic nucleus, are made of smaller pieces called "quarks." Some types of quarks can form particles that exhibit surprising behaviors. Mark Mattson, Ph.D., assistant professor-research, ...

Explainer: What are leptons?

January 7, 2014

The giant Large Hadron Collider at CERN's lab in Europe may be closed until 2015 but experiments will still be run there in the second half of this year on much smaller synchrotrons that examined the decay of particles into ...

Recommended for you

Seeing quantum motion

August 28, 2015

Consider the pendulum of a grandfather clock. If you forget to wind it, you will eventually find the pendulum at rest, unmoving. However, this simple observation is only valid at the level of classical physics—the laws ...

A little light interaction leaves quantum physicists beaming

August 24, 2015

A team of physicists at the University of Toronto (U of T) have taken a step toward making the essential building block of quantum computers out of pure light. Their advance, described in a paper published this week in Nature ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.