Antiprotonic helium

An exotic atom in which an electron and an antiproton orbit a helium nucleus could reveal if there are any differences between matter and antimatter. John Eades describes this unusual mirror on the antiworld. Atomic hydrogen and helium have played crucial roles in the development of quantum theory for more than a century. As the first two elements in the periodic table, with just one and two electrons respectively, they have been the easiest atoms to deal with mathematically, and this has allowed theory and experiment to be compared with ever-increasing precision. In recent years, however, these two basic atoms have been joined in the laboratory by two exotic cousins: antihydrogen and antiprotonic helium. Whereas antihydrogen is the antimatter equivalent of hydrogen - an 'anti-atom' that contains a positron orbiting around an antiproton nucleus - antiprotonic helium is part matter and part antimatter, since it is a helium atom in which one of the electrons has been replaced by an antiproton. Antihydrogen is the simpler of these two exotic atoms and therefore the one that is best suited to the fundamental question of whether there is any real difference or asymmetry between matter and antimatter (see Probing the antiworld). However, it is much easier to make antiprotonic helium. Moreover, antiprotonic helium can also exist in metastable or long-lived states, which is why this rather strange atom has been used in these experiments for the past decade. In the next few years it should be possible to place limits on possible differences between matter and antimatter with antihydrogen, but until that happens, antiprotonic helium will continue to have this exciting area of physics to itself. (U.K.)