## Quarks

Enjoying the fruit in the orchard of elementary particles was not without its problems. In 1934, there were protons, neutrons and electrons. These were the fundamental building blocks of matter. We had a seemingly simple model of what the universe was made of. The picture suddenly changed and became anything but simple. It seemed incomprehensible that Nature should have myriads of fundamental entities, with a whole spectrum of masses from zero to larger than the proton, which had no visible structure.

It was up to the theoretical physicists to find order in the midst of apparent chaos. Murray Gell-Mann (1929-), Yuval Ne'eman (1925-2006) and George Zweig (1937-) of the California Institute of Technology were convinced that the principles of symmetry must apply to the world of the very small just as they do in classical physics. They used an algebra originally developed in the 19th century by a Norwegian mathematician, Marius Sophus Lie (1842-1899), to group the particles into mathematical families. Technically the symmetry was called

SU(3) (a special unitary symmetry group of arrays of size 3 x 3). In 1963, Gell-Mann suggested a conceptually easier description in which the three abstract mathematical entities were identified as real objects, which he named 'quarks'.

The quarks were given the somewhat peculiar names of u (up), d (down) and s (strange). Protons, neutrons and heavier particles are made of three quarks. So, for example, the formula for a proton is uud, and that for a neutron is udd. The antiproton and the antineutron consist of three corresponding antiquarks. Mesons are made up of one quark and one antiquark; for example, the constituents of the n~ meson are u- d.

All normal matter is made up exclusively of u and d quarks. The s quark is a constituent of particles that were discovered in the early 1950s and called 'strange' because their behaviour was not well understood at the time.

A total of 27 states can be constructed out of the 3 quarks. They are arranged as one decuplet, two octets and one singlet. Their properties corresponded well with properties of families of known particles, with one notable exception. The heaviest member of the decuplet, made from three s quarks, had not been observed. As one can imagine, a huge effort was made immediately to find the missing particle. Within a few months a new particle, named O-, was found at Brookhaven and its properties slotted it very neatly into the vacant place in the SU(3) decuplet. It seemed that the problem was solved, and in 1969 Gell-Mann received the Nobel Prize for 'classifying elementary particles'.

Gell-Mann had the sound in his mind before the spelling of 'quark', which rhymes with 'stork'. In his occasional perusals of

The quark model refers only to hadrons, the particles which are subject to the strong nuclear force. Leptons, such as the electron, the muon and the neutrino, have no sub-structure.

James Joyce's Finnegans Wake he had come across the following passage:

'Three quarks for Muster Mark'! Sure he hasn't got much of a bark, And sure any he has It's all beside the mark

Whatever about the pronunciation, the number 3 fitted nicely into his scheme, despite Joyce's obvious intent that it should rhyme with 'bark!'

The story of quarks and their literary connection was the product of the imagination of a physicist with wide interests and a sharp sense of humour. Whether quarks are real physical objects or abstract mathematical entities is not important; the essential thing is that the representation gives results and makes further predictions which agree with experiment.

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