Positronium pairing points to a new kind of chemistry.
Physicists suspect they have created the first molecules from atoms that meld matter with antimatter.
Allen Mills of the University of California, Riverside, and his colleagues say they have seen telltale signs of positronium molecules, made from two positronium atoms1.
Positronium is an other-worldly mimic of hydrogen. In a hydrogen atom, a negatively charged electron moves around a proton, which has a positive charge. The electrical force of attraction between the two subatomic particles holds them together.
In positronium, hydrogen's proton is replaced by a positron, which is the antimatter partner of an electron. A positron has the same positive charge as a proton, but has the same mass as an electron, which is just 1/1,836 that of a proton. So positronium is an extremely light 'atom'.
Just as hydrogen atoms will combine to form two-atom molecules, so it should be possible in theory to unite two positronium atoms to form a molecule, which would be written as Ps2.
But positronium does not exist naturally, because matter and antimatter annihilate one another when they meet, releasing a burst of energy. When a positron and electron are combined artificially to create a positronium atom, as was first done in 1951(ref. 2), it quickly self-destructs, emitting energy as a gamma ray.
Because positronium is so unstable, it is hard to make a positronium gas dense enough for the atoms to link together. Still, Mills and colleagues think they may now have found a way, and suspect they have seen a hint that positronium molecules existed briefly before self-destructing.
Scientists have made other bizarre constructions from antimatter before. Atoms of antihydrogen, made up of the antimatter equivalents of protons and electrons, have been made in the lab. And researchers in Denmark made single positronium atoms link up with ordinary hydrogen atoms in 1992, forming the molecule PsH (ref. 3). They have even speculated about making 'positronium water': Ps2O.
But if Mills's positronium molecules prove to be real, this will be 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.
In the experiment that made the weird molecules, Mills and his co-workers fired a beam of positrons, emitted in the decay of a radioactive form of sodium, at a target of ordinary silica, like window glass full of tiny holes. In the collisions, some of the positrons picked up an electron to form positronium. The positronium atoms accumulate inside the porous silica, where their density may become high enough to allow Ps2 to form.
The team was able to monitor the amount of positronium that accumulated by measuring the intensity of the gamma rays generated when the electrons and positrons annihilate.
The researchers expected this intensity to be higher from the silica than from isolated positronium atoms, because matter and antimatter would be expected to whack into each other more often when confined to close quarters. But it turned out that the intensity was even greater than predicted. This, they say, could be because some of the positronium atoms are combining into molecules, which raises the chances of their electrons and positrons running into one another.
That is not the only possible explanation for the results, however. It may be that the greater rate of destruction comes from an extra compression of the positronium gas in small cracks and flaws in the porous silica. The researchers hope to distinguish between these possibilities in future experiments by using a different target.