Oily trap for a quark

If caught, it could spell trouble for the Standard Model of particle physics; although success has eluded most scientists, a group of relentless researchers refuse to give up the search for a free quark

Published: Friday 31 May 1996

The rate at which a drop falls (Credit: Anand Singh Rawat)A SMALL drop is threatening the whole ocean of thought that physicists have nourished for quite some time now. Most particle physicists believe that the theory which governs the interaction of matter at the smallest scale, the so-called Standard Model of particle physics, has Deen tested to a reasonable degree of accuracy. Martin Perl, co-recipient of the 1995 Nobel prize in physics and his colleagues differ.

Ped's objective: upsetting the Standard Model. His experimental arrangement: tiny droplets of silicone oil, about seven micrometres (one inicrometre- 10-6 metre) in diameter, falling between two electrically charged plates lit by a strobe light and a charge- coupled device camera to record the images at the Stanford Linear Accelerator Centre, us. His principle: the same that Robert Millikan applied in his classical oil drop method to evaluate the charge on a single electron, that is, the rate at which each drop falls through the varying electric field indicates the charge it carries. The success indicator is finding a droplet with charge less than that on an electron, the amount that a quark, fundamental particles that make up electrons and protons, would carry on its own.

There has been no success until now which is in confirmation with the concept of quantum chromodynamics (QCD), one of the components of the Standard Model. The basis of QCD is the existence of quarks which are bound together by strong forces. Quarks differ from other particles because they carry fractional charge and exist only in groups of two's or three's. A free quark cannot exist. Or so says QCD.

Ever since Murray Gell Mann and George Zwieg proposed the quark model in 1964, several searches have been carried out for free quarks. The techniques used have varied from a modified Millikans oil drop experiment to magnetic levitation, particle accelerator experiments and searches in cosmic ray tracks. The materials searched have ranged from niobium to moon rocks brought back by the astronauts! But all these experiments, barring one, have come up with a negative result; none of them have seen a free quark.

'In 1968, Brian McCusker first reported a track in a cloud chamber (a kind of particle detector) which looked like it was produced by an unusual particle. Its density was much less than would be expected from a conventional particle like a proton. This report remained unconfirmed and the track was not found in the many cloud chamber pictures taken subsequently.

In the late '70s, William Fairbank reported seeing fractionally charged particles on niobium-coated tungsten balls in a repeat of the Millikan oil drop experiment. There was a lot of excitemerit in the scientific community but it fizzled out because the results could not be replicated by many others, who carried out similar experiments. The result was that most scientists gave up the search and were brought around to believing QCD.

Though no evidence of a fractional charge has been seen, Perl and his co-workers are now improving their apparatus to examine much more material in a much shorter time. They plan to use the modified arrangement to study other materials later this year. If successful, it could be a major blow to QCD and will send the theorists back to their calculations (Science, Vol 2 71, No 5254).

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