WHAT HAPPENS WHEN MATTER MEETS ANTIMATTER? Not content to look for annihilation radiation in the heavens, scientists have manufactured their own proton-antiproton and electron-positron collisions in the lab. But what about hydrogen and antihydrogen? Anti-H atoms have been made fleetingly in collisions at CERN and Fermilab but not in a way which allowed detailed study (Updates 366, 253), but this should change in the next few years when, at CERN's Antiproton Decelerator, slow antiprotons and slow positrons are brought together.

To prepare for this eventuality, Piotr Froelich of Uppsala University in Sweden (011-46-18-471-3262; piotr@kvac.uu-se) has studied how the four particles (the proton and electron inside the H atom and the positron and antiproton inside the anti-H atom) come together. While modeling this process, he found that when colliding at low speeds, H and anti-H have a tendency to recombine into protonium (proton plus antiproton, abbreviated Pn) and positronium (electron plus positron, abbreviated Ps) before the particles and antiparticles annihilate. Both Pn and Ps are highly unstable but are slightly longer lived than they would be in the absence of the other pairing; that is, the positronium helps to screen the proton-antiproton interaction.

Consideration of the delicate balance between the reactive collisions which lead to annihilation, and the non-reactive ones which leave the hydrogen-antihydrogen pair intact, leads Froelich to believe that atoms and antiatoms might suffer each other's presence longer than was thought possible. This will have a practical impact on the study of anti-H atoms in enclosures maintained at high, but not perfect, vacuum conditions. (Froelich et al., Physical Review Letters, 15 May /pnu/2000/; Select Article.)