The Track of the Quark

Probing ever deeper into the inner world of the atom, nuclear physicists have uncovered an increasingly baffling collection of tiny particles. Besides the familiar neutrons, electrons and protons, they are now pondering dozens of new and strange bits of matter bearing such exotic names as lambdas, pions, kaons and sigmas. Five years ago, in an effort to bring order to this subatomic chaos, Physicists Murray Gell-Mann and George Zweig, both now at Caltech, in dependently dreamed up strange elemental particles--out of which all the others could be constructed. Gell-Mann emphasized that the particles, which he whimsically dubbed quarks, were only theoretical tools, mathematical concoctions that probably did not really exist outside his equations. Yet other physicists took the quark quite seriously, and have been hunting for it ever since.

Elusive Particles. Last week, for the first time, there was evidence that the hunters were closing in on their quarry. At a conference of the International Union of Pure and Applied Physics in Budapest, a scientist from Australia announced that he was "99% sure" that he had actually found a quark. British-born Physicist Charles McCusker, 50, reported that his team of investigators had apparently spotted the elusive particles among the wreckage of atmospheric oxygen and nitrogen atoms smashed when they were struck by cosmic rays hurtling down from space.

A number of scientists had previously suggested cosmic rays as an ideal weapon to use in the quark hunt. If one of these high-speed bits of matter struck an atomic particle, they calculated, its tremendous energy would accomplish what no man-made atom smasher can do: split that particle into its constituent quarks. A particle with an energy of 200 billion electron volts, for example, might be enough to pry apart the three tightly bound quarks that theoretically constitute a proton. But a machine that can supply such energy will not be available until the AEC completes its giant accelerator at Weston, Ill.

Unwilling to wait, McCusker's team set up a more simple quark trap in a shed behind the University of Sydney's school of physics. Whenever Geiger counters detected a cosmic shower, they triggered four Wilson cloud chambers, which show the path of any ionized or charged particle that passes through them as a trail of condensed water drop lets. If a quark freed by a collision between a cosmic ray and an atmospheric atom happened to penetrate the cham ber, the physicists reasoned, it would leave a highly characteristic track.

McCusker's team photographed 60,000 tracks in a year of work. Most of them bore the easily identifiable signatures of known particles. But a few consisted of only about half as many water droplets as the others. That observation fitted neatly with a peculiarity of quarks: unlike ordinary particles, whose charges are whole multiples of an electron charge, quarks ought to have a charge only one-third or two-thirds that of an electron. McCusker's conclusion followed logically: The number of droplets in a cloud-chamber track is proportional to the square of the charge of the particle that caused it. If quarks have a charge of two-thirds, the number of droplets in their track should be four-ninths (two-thirds squared) or about half the number in the track of an ordinary particle. And that is just what McCusker observed about five tracks in his quark trap.

Most physicists, of course, would like to see more persuasive evidence before they accept the existence of quarks, and even the enthusiastic McCusker allows that his experiment is hardly the final word. Even so, his findings are already the hottest bit of shoptalk among nuclear physicists. "If they are quarks," says Columbia University Physicist Leon Lederman, "they would be one of the major discoveries of the century."

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