The Chemical World This Week
STANFORD FINDING: QUARK OR MAYDE A QUIRK? The title of the talk was carefully phrased: "Evidence for the Existence of Fractional Charge on Matter," by Stanford physicist William M. Fairbank and his student George S. LaRue. But no one at the Washington, D.C., meeting of the American Physical Society last week was under any illusion about what was meant, and in the end, even the cautious Fairbank was freely using the magic word: "quark." For 14 years, since it was first proposed by Murray Gell-Mann and George Zweig in 1963, the quark has been the Holy Grail of physics. Thought to be the fundamental building block of subatomic particles, it has been sought in everything from cosmic rays to seawater, without success. If found, a quark would be unmistakable: Various types would have electric charges either one third or two thirds that of the proton. Fairbank and LaRue have not found an isolated particle. Instead, in a modern version of Millikan's 1910 oil drop experiment, they have produced a tiny niobium sphere that consistently exhibits a charge one third the unit charge. Somewhere on that sphere there seems to be a quark. The sphere is one of eight tested so far in the experiment. Each is about one fourth the diameter of a ballpoint pen tip. During the experiment a sphere is cooled to liquid helium temperatures, becomes superconducting, and is levitated in a magnetic field between two capacitor plates. After the sphere is neutralized as much as possible by ions from a radioactive source, its motion in an electric field between the plates gives a measure of its residual charge. On all the spheres, Fairbank and LaRue can detect integral multiples of charge due to individual electrons. Last year, however, one sphere showed a charge of minus one third— which vanished when the sphere was handled at room temperature. The physicists think that the quark, if there was one, may have been attached to a surface atom that was rubbed off. Then, on Feb. 15, LaRue found another ball with +0.337 ± 0.009 times the electron charge. This charge has remained through two subsequent tests. Significantly, other balls 6
C&EN May 2, 1977
measured in between have continued to be neutral. Fairbank and LaRue are confident they have eliminated systemic error such as dipole effects and mirror charge corrections. But full peer review of their experiment has only just begun, and with so much at stake it is sure to be grueling. Some questions are disturbing physicists already. If it's been so hard to find quarks in other experiments, they ask, then why is it so easy in this one? Two scores out of eight tries is a pretty spectacular average in this game. Furthermore, some of the balls were treated differently than others.
The balls were annealed on metal plates: five on niobium, three on tungsten. The two fractionally charged balls were both in the tungsten group. "Fairbank has a high reputation as a careful experimenter," says Harvard's Steven Weinberg. "But there's not a great amount of data, and they must be confirmed." Lately physicists have begun to believe that quarks, though real in a mathematical sense, are unobservable in principle. If one really has been found, says Weinberg, "it's going to force a lot of people, including me, to change their ideas." •
North Sea well blowout
