Making Antihydrogen Cooler

Very cool. A beam of excited cesium atoms hits a trap full of antielectrons, and the products fall into a pile of antiprotons. The result: cold antihydrogens.

A clever new way to make antihydrogen may bring scientists one step closer to understanding how matter differs from antimatter.

For years, a handful of antimatter-makers known as the ATRAP collaboration and a rival group, ATHENA, have been cooling antiprotons (which come from a beam at CERN, the European particle physics lab near Geneva, Switzerland) and antielectrons (which come from a radioactive source) and mixing them in a magnetic bottle in hopes of producing antihydrogen. Both teams have created thousands of antihydrogen atoms this way (Science, 15 November 2002, p. 1327). However, those antihydrogens were relatively warm--several degrees above absolute zero--and, therefore, too fast to capture and study in detail.

ATRAP's new method collects antiprotons and antielectrons in separate magnetic traps. Then researchers shoot a beam of cesium atoms toward the antielectrons. As the cesiums fly, lasers excite the atoms, forcing their electrons into larger-than-typical orbits around the cesium nucleus. These larger orbits increase the odds that a cesium atom will strike an antielectron in the trap, says Harvard physicist and ATRAP member Gerald Gabrielse.

After impact, the cesium's electron binds to an antielectron, forming an unstable and excited conglomerate known as positronium. The positroniums zoom away in all directions, and some wind up in the nearby trap containing antiprotons. Following another collision, the antielectron once again jumps ship and hops to the antiproton, forming an excited antihydrogen.

This Rube Goldberg-ish method has so far produced fewer than two dozen antihydrogens, the team reports in the 31 December issue of Physical Review Letters. But in principle, it should allow physicists to create very cold and slow-moving antihydrogens, which would be easier to study.

"Anything that goes in this direction is welcome," says Rolf Landua, a CERN physicist and member of the ATHENA collaboration. But the low yield is a problem, he cautions, and studying the produced antihydrogen properly will likely require deexciting the atoms, perhaps with another laser. "Maybe, in the end, that will be the way forward, but it looks complicated," Landua says.

Unfortunately, scientists will have to wait to find out. The antiproton source at CERN has been shut down until 2006 to speed construction of the Large Hadron Collider.

Related sites
CERN site on antimatter

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