The Standard Model of particle physics says that every particle has an antimatter counterpart – the electron, for example, is paired with the positively charged positron. Although electrons and positrons annihilate each other, they can bind together temporarily to create a positronium atom, which resembles a hydrogen atom. In theory, two positronium atoms could join to form a dipositronium molecule. However, physicists had found it hard to make detectable quantities of dipositronium because it is very difficult to get enough atoms in the same place to react and form molecules.

Now, David Cassidy and Allen Mills of the University of California at Riverside have managed to collect and react enough positronium to confirm that dipositronium exists. The pair used a special positron trap developed by Clifford Surko and colleagues at the University of California at San Diego to collect positrons from the decay of sodium-22.

When about 20 million positrons were accumulated, the contents of the trap were focused onto a small spot on a piece of porous silica. The positrons made their way into the pores, where they reacted with electrons to form positronium. Some of these atoms stick to the surfaces of the silica, where they combine to form dipositronium. The surface plays a crucial role in encouraging the dipositronium to form because it stabilizes the molecules by absorbing energy that is given off when the molecule is formed.

The presence of dipositronium was confirmed by keeping an eye on electron-positron annihilation in the silica. Positronium atoms exist in two different quantum states depending on the relative orientation of the electron and positron spins. The “para” state only lasts about 125 ps before annihilating, while the “ortho” state hangs on for more than 1000-times longer (142 ns) before annihilating. Dipositronium is formed when two ortho atoms come together, but there is nothing to stop the two positrons in the molecule from exchanging their electron partners and creating para atoms. As a result, ortho atoms in molecules don’t last as long as free ortho atoms.

By monitoring the gamma rays that are given off during annihilation, researchers saw a reduction in the overall lifetime of positronium in the silica, which they interpreted as evidence for the formation of dipositronium. According to Cassidy, this was confirmed by heating the silica, which prevents positronium from sticking and reduces the number of dipositronium molecules that can be created. When this was done, the lifetime of the positronium increased.

Cassidy told physicsworld.com that he and Mills are now working on creating a Bose-Einstein condensate (BEC) of positronium, in which all the molecules settle into the same quantum state. Calculations suggest that BEC could be made by boosting the density of positronium by a factor of 1000 and cooling it to about 15 K. Cassidy says that this could be done by accumulating more positrons in the trap and then firing a more intense beam at the silica. Improvements to the silica itself could also help, he says.

If the density were increased by another factor of 1000, the BEC could be used to create an annihilation gamma-ray laser. In such a device the positron/electron pairs could be made to annihilate in a cascade, which would produce a stream of coherent gamma-ray photons resembling laser light. Annihilation gamma rays have a very short wavelength, which means that such a laser could someday be used to study objects as small as atomic nuclei.