Making antihydrogen atoms at Fermilab.

Creating hydrogen is as straightforward as mixing together electrons and protons so that electrons wind up orbiting protons. Making antihydrogen is much tougher because the ingredients-positrons (the positively charged antimatter counterparts of electrons) and antiprotons (the negatively charged antimatter counterparts of protons)-are significantly trickier to obtain, store, and control (SN: 10/21/95, p. 268).

Not until last fall did physicists produce antihydrogen in the laboratory for the first time (SN: 1/13/96, p. 20). Now, a second team of researchers has reported creating antihydrogen atoms, which they generated in high-speed interactions between a beam of antiprotons and a jet of hydrogen gas.

David C. Christian of the Fermi National Accelerator Laboratory in Batavia, Ill., and his coworkers announced their initial findings last week, reporting the detection of seven antihydrogen atoms.

In both the earlier experiment at the European Laboratory for Particle Physics (CERN) in Geneva and the Fermilab experiment, researchers generated beams of antiprotons traveling at nearly the speed of light in an accelerator. Periodically, the circulating antiprotons would pass through a transverse jet of gas atoms. The CERN team used xenon for its jet, whereas the Fermilab group chose hydrogen.

Occasionally, an antiproton would pass close enough to a jet atom to give up a portion of its energy to create an electron and a positron. In even rarer instances, the newly created positron had a velocity close to that of an antiproton, so the position would get captured, creating a short-lived atom of antihydrogen.

Taking advantage of a more intense, higher-energy antiproton source than that at CERN, the Fermilab team expects to create and detect as many as five antimatter atoms per day once the accelerator is running optimally. "We anticipate detecting about 750 atoms of antihydrogen by next September," says Glenn D. Blanford of the Fermilab team.

Physicists would like eventually to use antihydrogen to check whether antimatter behaves in exactly the same way as ordinary matter. One crucial test involves a precise comparison of the wavelengths of light absorbed and emitted by these atoms.

However, the antiatoms created at CERN and Fermilab travel far too quickly and don't last long enough for researchers to measure their characteristics. Only when scientists can trap a large number of antihydrogen atoms can such investigations proceed.