But if larger forms of anti-matter such as anti-atoms, anti-stars and anti-galaxies were plentiful in the visible part of the universe, we would see the catastrophic gamma ray glare from places where matter brushes up against antimatter. Such radiation has not been seen and scientists must make their own anti-atoms artificially.

Making antihydrogen is difficult, however, because positrons and antiprotons, even when they can be marshaled and brought near each other, are usually going past each other too quickly for neutral atoms to form.

A few years ago, a dozen or so hot antihydrogen atoms were made on the fly amid violent scattering interactions at CERN and Fermilab. These did not dally long enough to be studied, but instead expired quickly when they crashed into detectors that established the antihydrogen's brief existence.

Now, cold antihydrogen atoms might have been made, for the first time, in an experiment at the CERN lab, where positrons and antiprotons are brought together in a bottle made of electric and magnetic fields.

At CERN, several experiments are devoted to making cold anti-atoms in a controlled environment amenable to detailed studies. The main goal here is to determine whether the laws of physics (gravity, quantum mechanics, relativity, etc.) apply to anti-atoms the same as they do to regular atoms.

At last week's meeting of the American Association for the Advancement of Science (AAAS) in Boston, Gerald Gabrielse of Harvard, spokesperson for the Antihydrogen Trap collaboration (ATRAP), reported new results. In his experiment, 6-MeV antiprotons (themselves made by smashing a beam of protons into a target) are slowed by a factor of 10 billion (to an equivalent temperature of 4 K), partly by mixing them with cold electrons, and then collected in a trap. Positrons from the decay of sodium-22 nuclei are cooled and collected at the other end of the device.

Eventually, about 300,000 positrons are electrically nudged into the vicinity of about 50,000 antiprotons.

Gabrielse believes that what sits in his trap isn't entirely a neutral plasma consisting of coincident positron and antiproton clouds, and that cold antihydrogen atoms might have formed.

More diagnostic equipment being installed now may settle the issue in the coming months. A larger version of the ATRAP apparatus, which might be in operation as early as this fall, should allow the researchers to introduce some lasers for the purpose of studying the spectroscopy of prospective anti-hydrogen atoms in the trap.

(Editor's Note: This story is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 577, February 20, 2002, by Phillip F. Schewe, Ben Stein, and James Riordon.)

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ATRAP

25-Feb-2002