Physics with antiparticles is difficult, but one trick is to replace atomic electrons by antiprotons. The resulting compact atoms are useful antiparticle laboratories.
Antiprotonic atoms, in which an antiparticle is bound to an ordinary nucleus, carry important messages about the antiworld and are much easier to make than anti-atoms. Among antiprotonic atoms, protonium (a “nuclear” proton and an “orbital” antiproton) is particularly interesting because it is the simplest two-body system consisting of a strongly interacting particleantiparticle pair.
An isolated protonium atom will not be destroyed by collisions with atoms of the medium in which it was produced and can only de-excite by giving off radiation. The lifetime can then easily exceed microseconds. The difficulty will be to produce the atoms in isolation.
Antiprotonic helium is a special case. An experiment at CERN discovered that this exotic atom can survive a very large number of collisions and survive long enough to be studied by laser spectroscopy.
Isolated antiprotonic lithium would also be of great interest because its antiproton orbit should be far outside the residual pair of electrons. It should then be able to descend a ladder of these slow electromagnetic transitions, which ends only when it approaches the electrons.
In studying the interactions of antiprotons with matter, it is important to understand their ionization effects how antiprotons strip electrons from ordinary atoms.
An experiment at LEAR by a collaboration involving Aarhus, PSI Villigen, University College London and St Patrick’s Maynooth measured the ionization of hydrogen by antiprotons within the 30-1000 keV energy range, where the antiprotons can be considered to be “fast and heavy” (see next article). The experimentally observed effects concur with theoretical calculations.
However, at lower energies, where there are as yet no data, theoretical analysis becomes more difficult and different calculations disagree, although they suggest an at the most weak energy dependence.
The study of the ionization of helium by antiprotons, with removal of one or both electrons, was pioneered at LEAR and is also ripe for further investigation.
These additional physics objectives form an integral part of the ASACUSA experimental programme, which involves some 50 researchers from 19 research institutes and in which Japanese physicists play a prominent role.
Antiprotons cannot do this, but when their energy drops still further (below a few tens of electron volts) they will readily be captured by the nucleus (see previous article) and form antiprotonic atoms.
These effects showed up clearly in the very-low-energy domain of antiproton physics opened up at CERN’s LEAR low-energy antiproton ring, and groups from Aarhus and Tokyo carried out many atomic interaction experiments as a guide to a better theoretical understanding of these many-body collisions (see previous article).
In the LEAR era, such experiments injected high-energy antiprotons into metallic foils or high-density gases, which degraded the antiprotons to electron volt energies and (in some experiments) provided the target atoms in which they were finally captured.
If the target density or thickness could be made so small that only one collision occured, much more precise and better-controlled experiments on the atomic interactions of antiprotons would be possible, and the dynamics of antiprotonic atom formation could be studied in detail. At such low target densities the absence of collisions after the capture process should also ensure that all antiprotonic atoms are stable enough to be brought under the penetrating eye of laser spectroscopy (see previous article).
The thin-target condition, where a beam particle enters a target and makes a single interaction, is, in a sense, “business as usual” for high-energy particle experiments, yet it constitutes one of ASACUSA’s more difficult longer-term goals. The solution is to separate the deceleration of the antiprotons from the atomic interaction (or antiprotonic atom formation) to be studied.
However, the electron volt antiprotons required for these experiments have a millionth of the energy that even the AD can provide. This energy gap will be crossed in two stages. First, the AD will be supplemented by a decelerating Radio Frequency Quadrupole (under construction in CERN PS division) to reduce the energy to tens of kilo electron volts. The antiprotons will then be confined in a Penning trap that is being constructed at Tokyo University, cooled to cryogenic temperatures, and reaccelerated to a given electron volt-scale energy.
Finally, the reaccelerated antiprotons will be introduced into low pressure gas targets or jets or ultrathin foils. These experiments should start in 2000, after the first round of experiments (on antiprotonic helium) is complete.
