Exotic adventures en route to the antiworld

Vladimir Korobov of the Joint Institute for Nuclear Research at Dubna in Russia and Dmitri Bakalov of the Institute for Nuclear Research and Nuclear Energy in Sofia, Bulgaria, have made a theoretical breakthrough in interpreting the properties of antiprotonic helium atoms (helium atoms in which an electron has been replaced by an antiproton) in terms of the properties of the antiproton itself. Classic quantum mechanics suggests that when such a atom is highly excited, the orbiting antiproton should quickly decay, emitting visible photons as it goes. One crucial defect in this argument is its neglect of the force between the antiproton and the remaining electron. More detailed calculations suggest that the antiproton will not have time to decay in such fashion, or emit any photons. So it came as a shock in 1990 when a group lead by Toshimitsu Yamazaki of Tokyo University showed that about 3% of all antiprotonic helium atoms survive for several microseconds in spite of the antiproton/electron interactions. This metastability against annihilations appears to be connected with a delay in the ejection of a second electron, during which time the antiprotonic atom continues to live. Furthermore, the electron seems to reduce the atom's sensitivity to collisions. This was confirmed in 1993 when the Japanese group, now joined by several European colleagues, showed that antiprotonic helium could indeed be stimulated by a laser beam to make a quantum jump of about 2 eV, from an n=39 level to an n=38 level (N Morita et al. 1994 Phys. Rev. Lett. 72 1180). This result highlights the importance of the Korobov and Bakalov paper. Since the second electron is essential for this metastability, its small but non-negligible interaction energy with the antiproton must be taken into account in calculations of energy levels. (UK)