An Antimatter Molecule?

by Paul Gilster on November 28, 2005

With Hayabusa apparently stabilized and ready to begin its return journey to Earth, and with the Falcon-1 launch delayed until mid-December, it’s time to return to research. But not before congratulating the Japanese space agency (JAXA) for the probe’s apparent success in landing on the asteroid Itokawa, collecting surface samples, and lifting off again. These would be the first asteroid materials ever returned to Earth, and if their landing in 2007 proceeds as planned, they will be the capstone of a remarkable mission.

On the research front, what catches the eye this foggy North Carolina morning is the report in Nature that scientists may have created positronium molecules made out of two positronium atoms. If so, it would be a singular accomplishment. Positronium replaces the hydrogen proton with a positron (the antimatter equivalent of an electron). So instead of normal hydrogen’s single electron moving around a proton, you get an electron moving around a positron which, like the proton, is positively charged, though far less massive. A team led by Allen Mills (University of California at Riverside) believes it may have seen double positronium molecules, each made up of two electrons and two positrons.

Needless to say, we are talking about an extremely unstable molecule since matter and antimatter annihilate when they meet. If the work stands up to scrutiny — and there are other explanations for these results — it would be the first time positronium molecules were observed to have a brief existence before their destruction and, as Nature opines, “…the first evidence for a new kind of chemistry, resulting from reactions between ‘explosive’ atoms that have a completely different physical make-up from those in nature.”

The experiment worked this way: the researchers targeted a surface made of porous silica with positrons. The positrons combined with electrons to form a concentration of unstable positronium atoms which moved into the pores of the silica and, colliding with each other, produced energetic gamma radiation. More about the experimental method can be found in a UCR press release, from which this:

“This is the first time anyone has been able to observe a collection of positronium atoms that collide with one another,” Mills said. “We knew we had a dense collection of these atoms because, being so close to one another, they were annihilating faster than when they were just by themselves.”

But Mills notes that the evidence for double positronium molecules in this experiment is only a ‘suggestion.’ Up next is the confirmation of their existence and the measurement of their actual properties. A paper by Mills’ team will appear next month in Physical Review Letters.