Per Ardua ad ASACUSA

At CERN’s AD Antiproton Decelerator, the ASACUSA collaboration is already preparing to greet the first AD antiprotons with a barrage of laser and microwave beams. ASACUSA stands for Atomic Spectroscopy And Collisions Using Slow Antiprotons, and, as this name implies, the experimenters’ joblist will include studies of the interaction of antiprotons with atoms at super-low energies, both as a means of understanding the formation of antiprotonic atoms, and as a subject in its own right.

Most physicists learn early in their career that it is impossible to find exact solutions for problems with more than two interacting bodies. Unfortunately, nature’s arrangements do not include making life easy for physicists ­ most of the phenomena that they find interesting (including those mentioned above) turn out to involve three bodies or more. Often physicists can avoid this handicap, sometimes by taking advantage of the fact that the masses and/or energies of some bodies may be much larger or smaller than those of other bodies; sometimes by using approximation methods; and sometimes by employing both approaches.

The many-body problem of the interaction of charged particle projectiles, such as protons and antiprotons, with atoms has repeatedly engaged many of the most agile minds of 20th-century physics. If, in such collisions, the incident particle is much heavier than the electrons in the target atom and its encounter with the atom is short- lived enough to be treated as a small perturbation, it will follow a straight, charge-independent, constant-velocity path through the atom and will not be deflected by electric fields.

This approximation, together with a few additional assumptions (for example, that the nucleus is too small a target to play a significant role), leads to the familiar Bethe­Bloch formula for the cumulative energy loss from multiple atomic encounters of charged particles passing through matter ­ of everyday importance in every particle physics experiment.

The “fast and heavy” approximation can at best hold down to projectile velocities about equal to that of the target atom’s electrons: about 25 keV for nucleons approaching hydrogen atoms. At lower energies the charge independence assumption will also be lost, because the projectile stays in the atom long enough to feel the nucleus. Among the more dramatic ultralow-energy effects is that of projectile protons repeatedly capturing and losing electrons.