Electric antimatter: checking the charge.

The skeptical, inquiring physicist takes nothing for granted. Consider, for example, the fundamental notion that electrical charge comes in packages of only a certain size and the complementary idea that the electron and proton have equal but opposite charges.

These notions date back to the beginning of the century. However, according to a suggestion made by Albert Einstein in 1924, a slight difference in the charges carried by the electron and proton could account for the existence of the magnetic fields of the sun and Earth. Other theorists have suggested that even a difference as small as one part in [10.sup.20] could account for the expansion of the universe.

Such considerations led to a number of experimental tests, which established to very high precision that the electron and proton charges are equal. Physicists have long assumed that the same equality holds between the electron and positron (the electron's positively charged, anti-matter counterpart) and between the proton and antiproton.

In the July 27 PHYSICAL REVIEW LETTERS, two researchers provide the first indication based on experimental data that this assumption is justified.

"We realized that there had never been any experiments to test that the quantum of charge on either the positron or antiproton is the same as the quantum of charge carried by the electron or proton. Nobody had questioned this at all," says Richard J. Hughes of the Los Alamos (N.M.) National Laboratory, who coauthored the new report with B.I. Deutch of the University of Aarhus in Denmark.

To rectify this long-standing oversight, Hughes and Deutch studied existing experimental data and found a way to combine the results from two different types of experiments to provide a direct, independent measure of the ratios of the electron-positron and proton-antiproton charges.

"Our results ... represent the first tests of charge quantization for the positron and antiproton, which, with the available experimental results, are at a much lower precision than the tests for electrons, protons, and neutrons," the researchers write.

Efforts to improve the precision hinge on planned experiments involving antihydrogen, which consists of a positron and an antiproton. "Although antihydrogen has not yet been produced in the laboratory, plans to do so and to perform precision spectroscopic measurements on it exist," Hughes and Deutch note.

"This is the kind of consistency check we need to do to answer how it is that we know certain things, such as conservation of charge in reactions," Hughes says.