Alliance of Opposites: Electrons and positrons make new molecule

By soaking a silica sponge with antimatter, physicists have made the first matter-antimatter molecules. With further refinement, the technique might be used to briefly condense antimatter into fluid or solid states or even to create the first gamma-ray laser.

About 10 years ago, researchers created atoms of antihydrogen by combining antiprotons and positrons, the antimatter equivalents of protons and electrons. By itself, antihydrogen is as stable as hydrogen, though it’s difficult to store in our matter world because of antimatter’s propensity to vanish in a flash of gamma rays as soon as it comes into contact with matter.

For more than 50 years, however, physicists have been able to create nucleus-free “atoms” consisting of one electron and one positron. Attracted by their opposite charges, electrons and positrons will orbit each other, as the stars in a binary system do.

Unlike antihydrogen, however this unusual matter-antimatter hybrid, called positronium, is unstable. It enjoys just a brief dance of death as the two particles spiral in toward mutual annihilation.

Still, positronium can live long enough—up to hundreds of nanoseconds—that physicists had speculated that the atoms might be able to pair up into molecules. Coaxing the atoms to do so would require assembling them in tight quarters and slowing them down enough to allow them to intermingle.

To perform this feat, David Cassidy and Allen Mills of the University of California, Riverside began by trapping millions of positrons—produced by a radioactive source—in an electromagnetic field. By applying brief electric pulses, the team expelled short bursts of positrons, directing them toward a thin, porous silica membrane. Inside the pores, some of the positrons scooped up electrons from the silica to form positronium.

The researchers hoped that some of the atoms would bounce around inside the pores and even temporarily stick to the pores’ inner surfaces, where feeble electrostatic forces might slow them down and allow them to bind to each other as molecules.

All the positrons, whether free or bound in atoms or molecules, eventually annihilated, producing gamma rays. But Cassidy and Mills detected a telltale gamma-ray signal that they had expected the annihilation of molecular positronium to produce. For confirmation, they heated the membrane, creating conditions that would prevent the formation of molecules. Sure enough, the signal disappeared, the team reports in the Sept. 13 Nature. Mills says that the data show “all the hallmarks” of the appearance of positronium molecules.

Clifford M. Surko of the University of California, San Diego says that the evidence for the formation of positronium is convincing, if indirect. “I did not find any obvious potential flaw in it,” he says.

This achievement is only the beginning, Mills says. If the researchers manage to concentrate more positrons into their sponge, more-complex states of matter should appear. In a Bose-Einstein condensate, an exotic gas in which atoms share a quantum state, positrons could be forced to annihilate in sync to produce the first gamma-ray laser, Cassidy says. Even higher densities could lead to the first solid matter–antimatter state.

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