Prying apart antimatter. (Physics).

Physicists in Switzerland have taken the first peek inside atoms of antimatter. The new experiments, which probed antihydrogen atoms, show no sign that physical laws differ between this exotic matter and ordinary matter. The new observations set the stage for far more precise comparisons that researchers say will test the foundations of modern physics.

Prevailing theories hold that antihydrogen and hydrogen are identical except for having constituents with opposite electrical charges. An atom of antihydrogen comprises a positively charged positron--the antimatter twin of the electron--orbiting a negatively charged antiproton. Scientists made the first atoms of antihydrogen in the mid-1990s within an accelerator, but those particles moved too quickly to be closely studied.

In October, a team of researchers at the European Organization for Nuclear Research (CERN) in Geneva reported making the first slow-moving antihydrogen atoms (SN: 11/2/02, p. 286)--a step toward high-precision measurements of the atoms' properties.

Now, a second CERN group, known by the acronym ATRAP, has independently made slow-paced antihydrogen atoms. Plus, the ATRAP team took another step: They used an electric field to pry apart the exotic atoms' positrons and antiprotons. "That gives us a peek inside," says ATRAP spokesman Gerald Gabrielse of Harvard University.

By revealing how tightly joined those components were, the prying provided a measure of the so-called ionization energies of antihydrogen atoms at different levels of excitation.

That's the kind of data physicists need for even more precise comparisons between matter and antimatter. Gabrielse and his colleagues describe their findings in the Nov. 18 and Dec. 2 issues of Physical Review Letters. --P.W.