快猫短视频

This is a classic article from 快猫短视频鈥檚 archive, republished as part of our 50th anniversary celebrations

BURIED among the many incomprehensible pages of Finnegans Wake, what sounds like the pronouncement of an obscure Celtic award runs: 鈥淭hree quarks for Muster Mark鈥�. Appropriately, because modern nuclear physics is often as unintelligible as Joyce to ordinary mortals, the term 鈥渜uarks鈥� was recently borrowed by Professor Murray Gell-Mann of the California Institute of Technology to describe some hypothetical new elementary particles. An experiment performed with the big proton synchrotron at Brookhaven now indicates that quarks do not exist. Or, to be precise, if they do exist they must have very large masses by the usual standards of particles.

Professor Gell-Mann is one of the originators of the Unitary Symmetry theory which has come to prominence as a result of its success in predicting the existence of the omega-minus particle. According to the theory, all the known 鈥渟trongly interacting鈥� elementary particles can be arranged into families in a way that interrelates their properties. There is one family of 10 which includes the omega-minus, and the algebra used also suggests that the fundamental family of particles from which all the others can be built up should contain just three members.

None of the known particles fits the bill and this led Professor Gell-Mann to postulate his quarks. Independently, G. Zweig, also of Caltech but currently working at the European Organisation for Nuclear Research (CERN), has put forward a similar theory 鈥� only he calls his triad of particles 鈥渁ces鈥�.

The really odd thing about quarks or aces was that they were supposed to have fractional electric charges. Ever since Millikan in 1911 did his ingenious oil-drop experiment to measure the size of the charge on the electron it has been part of the physicists鈥� gospel that the electron carries a basic unit of charge which is indivisible. Professor Gell-Mann鈥檚 three quarks were to be similar to the proton, neutron and lambda-particle but with charges of 2/3, 1/3 and -1/3 respectively of the usual unit charge. At least one of them was expected to be stable, so there was a chance of detecting it. A particle with a charge of 1/3 would only have one-ninth of the ionising power of an electron so that any track it left behind in a bubble chamber would have one-ninth the number of bubbles per centimetre for a given particle momentum. It would only emit one-ninth as much light in a scintillation detector.

L. B. Leipuner and his co-workers at the Brookhaven National Laboratory set out to look for events of this latter kind (Physical Review Letters, vol 12, p 423). Their statistics show that if any quarks with ordinary masses exist they are extremely rare 鈥� less than one for every few thousand million regular charged particles. At CERN, researchers have been scanning previous bubble-chamber photographs and an experiment has been running to look for quarks, again with negative results so far. The possibility remains that fundamental particles with fractional charges may still exist, but they would need to have masses more than three times that of the proton and to generate them would require bigger machines than either the CERN or Brookhaven accelerators.

This article was originally published in 快猫短视频 on 23 April 1964

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