One of the most exhaustive searches to date has failed to turn up evidence of free quarks. Quarks are tiny particles that make up the more familiar proton and neutron in the nucleus of the atom. Together with gluons -- particles that manifest the forces that bind them together -- quarks are the most fundamental building blocks yet discovered of the physical universe.
It is an open theoretical question whether a single quark can exist in isolation. Quarks normally associate in groups of three. Trios of different quarks make up the proton, neutron, and a host of other subatomic particles.
But, according to some theories, these infinitesmal objects are so tightly stuck together that they cannot be separated, regardless of the size of the ``hammer'' used.
Nothing is certain, however. Since the quark's existence was first suggested in 1964, there have been a number of attempts to detect them. They should have several distinctive characteristics, in particular an electrical charge a fraction of the electron's. But the results have been exasperating. Some experimenters, most notably William Fairbank of Stanford University, have reported evidence for fractional charges while many other efforts have turned up negative results.
The existence or non-existence of free quarks is primarily a matter of scientific concern. But there is at least a theoretical hint of a practical use for free quarks should they ever be found. Several years ago, George Zweig of the California Institute of Technology (Caltech) suggested that free quarks might be used to catalyze nuclear fusion.
Dr. Zweig's calculations indicate that, in the presence of free quarks, the nuclear reactions that power the sun could be coaxed to run at room temperatures and pressures, creating an intrinsically safe and nearly inexhaustible energy source.
This science-fiction-like possibility looks less likely this week, however. At the annual meeting of the American Physical Society, Robert D. McKeown, an assistant professor of physics at Caltech, reported the results from an experiment capable of detecting one quark among five hundred million billion more mundane particles.
Dr. McKeown and his colleagues used an ion beam to sputter samples of niobium and tungsten into a beam of individual atoms. The two metals were used because they were involved in Dr. Fairbank's experiment. In a Caltech accelerator, these atoms were accelerated to high energies and passed through a device that deflected them by an amount dependent on their electrical charge. No signals indicating fractional charges were detected.
``Our negative results certainly do not mean that we can rule out Dr. Fairbank's finding, but they do put a great many constraints on what one would assume he is observing,'' comments McKeown.
The Caltech scientists plan to extend their search to other substances.
Next, they will try copper wires that have been exposed to high-energy beams of heavy ions in a large particle accelerator.
And then they would like to search meteoritic material.