US team creates 'atom' containing antimatter

Posted by Kate Taylor

Physicists at the University of California, Riverside say they've made what could be a big breakthrough in the understanding of antimatter.

They've discovered a new way to create positronium, which is made up of an electron and its antimatter twin, the positron.

Antimatter's elusive stuff. Recently, scientists at CERN trapped antihydrogen atoms for more than 15 minutes, the first time it had ever been captured for longer than fractions of a second.

The UC Riverside physicists first irradiated samples of silicon - chosen because it has wide application in electronics, and is tough and cheap - with laser light. Next they implanted positrons on the surface of the silicon, and found that the laser light frees up silicon electrons that then bind with the positrons to make positronium.

"With this method, a substantial amount of positronium can be produced in a wide temperature range and in a very controllable way," says David Cassidy, an assistant project scientist in the Department of Physics and Astronomy. "

Other methods of producing positronium from surfaces require heating the samples to very high temperatures. Our method, on the other hand, works at almost any temperature – including very low temperatures."

When positrons are implanted into materials, they can sometimes get stuck on the surface, where they will quickly find electrons and be annihilated.

"In this work, we show that irradiating the surface with a laser just before the positrons arrive produces electrons that, ironically, help the positrons to leave the surface and avoid annihilation," says Allen Mills, a professor of physics and astronomy.

"They do this by forming positronium, which is spontaneously emitted from the surface. The free positronium lives more than 200 times longer than the surface positrons, so it is easy to detect."

In the long term, the researchers' hope to perform precision measurements on positronium to improve understanding of antimatter and its properties, as well as how it might be isolated for longer periods of time.

The next stage is to try to to cool the positronium down to lower energy emission levels for other experimental uses, and create a Bose-Einstein condensate for positronium – a collection of positronium atoms that are in the same quantum state.

"The creation of a Bose-Einstein condensate of positronium would really push the boundaries of what is possible in terms of real precision measurements," says Cassidy.

"Such measurements would shed more light on the properties of antimatter and may help us probe further into why there is asymmetry between matter and antimatter in the universe."