13 October 2006 Stephen Battersby Mixing antimatter and matter usually has predictably violent consequences – the two annihilate one another in a fierce burst of energy.
But physicists in Geneva have found a new way to make the two combine, at least briefly, into a single substance. This exceptionally unstable stuff, made of protons and antiprotons, is called protonium.
The feat of "antichemistry" actually took place back in 2002, but nobody had realised it until now. It happened in an experiment at the CERN particle physics lab, when both antiprotons and positrons – which have the same mass as electrons but an opposite charge – were put into the same magnetic cage. Some of them combined to make antihydrogen, which was the original aim of the experiment.
Now it seems that the same setup also produced a more peculiar, hybrid kind of matter, according to an analysis of the pattern of particle shrapnel flying out of the experiment.
Researchers led by Evandro Rizzini at Italy's University of Brescia believe that some of the antiprotons reacted with ionised molecules of ordinary hydrogen, stealing away a proton. These proton-antiproton systems lasted microseconds at most, but that was long enough for many of them to drift away from the core of the experiment before exploding.
Protonium has been made before, but only in violent particle collisions. The new chemical method could be used to make it in much larger quantities.
"The formation probability is very high," says Rizzini. With thousands of protonium atoms, it should be possible to study them more closely. Rizzini hopes they will help test current theories of particle physics – although other researchers say their brief lifetime could make that impossible.
Journal reference: Physical Review Letters (vol 97, no 153401)
