Antimatter persuaded to react with matter

Matter and antimatter usually destroy each other in a flash of energy and a spray of exotic particles when they meet. Yet the two have been coaxed into a chemical reaction by the international research group Athena. The product was protonium, a hybrid atom composed of a proton bound to an antiproton, its antimatter counterpart.

The experiment that created this unusual object was conducted four years ago in the particle physics laboratory Cern, in Geneva, Switzerland. The aim was to produce anti-hydrogen. As lead researcher Nicola Zurlo explains, the proton and electron each have an oppositely-charged antimatter counterpart, the antiproton and the anti-electron (or positron). Hydrogen is made of protons and electrons; anti-hydrogen is made of antiprotons and anti-electrons. 

In 2002, clouds of anti-hydrogen were made by delicately corralling anti-electrons and antiprotons together in a magnetic trap, in almost perfect vacuum conditions. But a few residual hydrogen ions, which could not be pumped out of the trap, must have engaged in their own exotic chemistry, as researchers realised upon looking more closely at the data.

Hydrogen molecular ions (H₂⁺) appear to have combined with antiprotons, to create a neutral hydrogen atom, and protonium. The protonium was detected after its death, via telltale charged particles that sprayed out when its constituent antiproton and proton collided together. 

Protonium has been created before by smashing antiprotons into matter, but not by a chemical reaction. The new method should allow the hybrid atom to be produced in much greater quantities. And because protonium is created in almost perfect vacuum, it survives for around a microsecond, a million times longer than if it were surrounded by matter.

That is just long enough for protonium’s energy levels to be probed by laser spectroscopy, says John Eades of Tokyo University, Japan, who feels that the Athena group have presented fairly convincing evidence for the hybrid atom’s creation.

High-precision spectroscopy on such unusual objects as anti-hydrogen and protonium tests fundamental theories of particle physics that describe the interactions between particles and antiparticles. Matter-antimatter interactions are already used as tools in medical diagnostics: in positron emission tomography (PET), for example, where positron-electron annihilations create gamma rays for computer analysis. 

Richard Van Noorden

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