Physicists create atom that has matter-antimatter

Under the standard laws of physics, for every type of ordinary-matter particle, a corresponding “antiparticle” exists.

For instance, the positively-charged proton has a negatively-charged counterpart, the antiproton. The electron, which is negatively charged, is offset by the positively-charged positron. When particles and antiparticles come together, the meeting is only very brief, for they annihilate each other in a flash of energy. In the case of electrons and positrons, the two species of particle create a short-lived, hydrogen-like atom called positronium, whose existence was first mooted in 1946 and confirmed five years later.

The theoretician behind positronium, American physicist John Wheeler, also suggested that positronium should exist as a two-atom molecule, called Ps2, and that there should even be a three-atom version, Ps3.

Until now, this hypothesis has never been confirmed, the big problem being the task of creating such finicky, fleeting molecules in lab conditions. In free space, two atoms of positronium cannot combine together, because they have such excess energy that they simply fly apart again. Writing in the British journal Nature, University of California Riverside physicists David Cassidy and Allen Mills describe how they overcame this obstacle to forge the world’s first Ps2 in laboratory conditions.

They did it by first creating a special trap that confined some 20 million positrons, which were then focused in a nano-second blast onto the surface of porous silica. Holed up within the pores, the positrons captured electrons to form atoms of positronium, which in turn linked up to form around 100,000 two-atom molecules before annilation.