The ASACUSA programme

ASACUSA stands for Atomic Spectroscopy And Collisions Using Antiprotons; spelt with a 'k' (as Asakusa) it is the name of one of the oldest quarters of Tokyo (right). Asakusa is not far from the Hongo campus of Tokyo university, which since 1991 has been closely involved in research on antiprotons at CERN. Initially these studies, in which laser beams were used to probe antiprotonic helium atoms, were carried out at the former Low Energy Antiproton Ring (LEAR); the new experimental programme was approved by the CERN Research Board in 1997 as the third element of the CERN AD (Antiproton Decelerator) programme.

Antiprotonic helium atoms, sometimes called 'atomcules', consist (right) of an antiproton p- and an electron e- bound by electrical forces to a helium nucleus He++, and belong to the same class of antiparticle-containing atoms as antihydrogen. They are extremely easy to create -- it is enough just to bring fast-moving antiprotons (previously produced by bombarding a metallic target with high energy protons) to rest in ordinary helium. The stationary antiprotons readily displace electrons from neighbouring helium atoms, and remain bound in their place, travelling around the helium nucleus in a slightly elliptical orbit. The sequence of collisions and other interactions with atoms of ordinary matter experienced by the antiprotons in coming to rest is an important research topic in itself, and was the subject of another LEAR project carried out by Aarhus University, Denmark. In these experiments, the detailed dynamics of atomic collisions of antiprotons was studied by measuring their energy loss in matter and the probability that electrons were ejected from atoms struck by them. The present collaboration includes laboratories involved in both these LEAR programmes together with several other research institutions throughout Japan and eastern and western Europe, and as its name suggests the ASACUSA programme now covers both these topics.

The significance of antiprotonic helium atoms

The LEAR antiprotonic helium experiments, in which laser beams were used to induce quantum jumps of the antiproton from one orbit to another, revealed that this unusual atom constitutes an extremely powerful microscope through which the antiproton can be studied in minute detail. The story began in 1991 at the KEK laboratory near Tokyo, where a Japanese team was following up an earlier observation that K^- mesons stopped in liquid helium took a longer time to be absorbed by the helium nucleus than expected. Repeating these measurements with antiprotons, they measured the elapsed time between the introduction of these particles into a liquid helium target and their subsequent annihilation. In about 3% of the cases, they found an average value of the order of 3mus (1 microsecond = one millionth of a second); for the remaining 97% it was about one picosecond (1 picosecond = one trillionth of a second), the value that had been confidently predicted for many years. Closer inspection at LEAR then showed that the longevity of the antiprotons could be attributed to the formation of a metastable (i.e. long-lived) form of the antiprotonic helium atom.

If antiprotons introduced into helium atoms had the expected picosecond lifetime they would orbit the helium nucleus only a few hundred times before inevitably falling into it and annihilating. In the case of the metastable atoms, the figure is about three billion orbits. To use an astrophysical metaphor, this is rather like believing that we inhabit a world in which all planets live only for a few hundred years before falling into their suns, and then discovering that a special class of planets like the earth exists, whose members survive for a significant part of the history of the universe.

The properties of any atom are determined by the properties of its constituent particles. It is the extremely long lifetime (in atomic terms) of these antiproton-containing helium atoms that permits their properties to be measured by the powerful and accurate tools of laser spectroscopy, and thereby gives them a kind of test-bench role for studying the antiproton itself. Already at LEAR the wavelengths of certain spectroscopic lines (which determine the energy required by the antiproton to jump between orbits) were measured to a few parts in ten million, permitting the antiproton charge and mass to be deduced with similar precision. Some of the LEAR results were reproduced at the AD within hours of delivering its first antiprotons (see figure)

First goals of ASACUSA at the AD

One of the main objectives of the ASACUSA programme is therefore to continue the study of the metastable antiprotonic helium atom at ever-higher precision as a test of the CPT-theorem -- the theorem behind our supposition that an antimatter world would be indistinguishable from the familiar one made of ordinary matter. These goals are exactly those of antihydrogen spectroscopy experiments. The latter has the advantage that antihydrogen's measured properties can be compared with those of ordinary hydrogen directly without needing to know to begin with what value is expected for either of them. Unfortunately antihydrogen is extremely difficult to synthesize, and this is indeed one of the main obstacles in the path of the other two AD experiments. While antiprotonic helium is, as we saw, extremely easy to synthesize, protonic antihelium -- the atom that would be needed for a direct comparison of the properties of the world and the antiworld -- is not. For the charge and mass measurements just referred to the experimenters therefore had to rely on having calculated values available for the measured spectral frequencies. Luckily computer calculation techniques had been developed (notably by Russian, Bulgarian and Japanese theorists) to provide these, and continue to advance so rapidly that antiprotonic helium is already a real alternative to antihydrogen for comparing worlds with antiworlds.

Later phases of experimentation

In a new phase of experimentation starting in October 2000, a radio frequency quadrupole will be used to decelerate the antiprotons further, until their energy has been reduced to a few tens of thousands of Volts (the AD itself only brings them down to about 5 million Volts). Production and use of these extremely low energy beams is technologically an extreme challenge. It has been taken up by the Aarhus group, who will study details of the antiproton's atomic interactions in this hitherto unexplored low-energy regime, in which it is moving even more slowly than the electrons contained in the atoms it is interacting with. The final phase (beginning in 2001) will be to collect samples of antiprotons in a trap and to extract them at still lower (eV) energies. These will make detailed studies of the formation process of antiprotonic atoms possible, as well as permitting the study of a metastable form of protonium p^--p.

FURTHER READING

General (Antiproton decelerator, antiproton physics:)

• Alexander Hellemans, Nature, August 2000, in press.
• J. Eades and F.J. Hartmann, 1999, Reviews of Modern Physics Vol 71, No 1 p 373.

Antiprotonic helium:

Metastable form discovered:

• M. Iwasaki et al., Physical Review Letters Sept. 1991, Vol67 p 1246
• J.Maddox, Nature, Sept 1991, vol 353, p207
• J. Eades, Europhysics News 1993 vol 24, p172
• R.J. Hughes Nature April 1994 Vol 368, p 813
• D. Horváth, "Atomcules: Long--living antiprotonic states in helium" Fizikai Szemle (Budapest) 44 (1994) 137--143 (In Hungarian).
• Corinne Ménard, Sciences et Avenir, October 1995 p 95 (French)
• Kagaku Asahi (Science Supplement, Asahi Shimbun), April 1994.

Laser spectroscopy:

• R. Hayano and N. Morita , Kagaku 1994 (Japanese)
• N. Morita et al., Physical Review Letters Feb 1994, Vol 72 No 8 p 1180.
• B. Ketzer et al., Physical Review Letters March 1997, Vol 78 No 9 p 1671.
• J. Eades, CERN Courier, April 1998 p 8 (French, English)

CPT-invariance, Antihydrogen etc. :

• J. Eades, Science Spectrum 1997, No. 8, p 7 (USA)
• J. Eades, Kagaku 1993, Vol 63 p 693 (Japanese)

Antiproton interactions with matter:

• J. Eades, CERN Courier, May 1999 p 16b (French, English)

Some technical papers:

• M. Iwasaki et al., Phys. Rev. Lett., 67 (1991), 1246-1249.
• T. Yamazaki et al., Nature 361 (1993) 238-240.
• N. Morita et al., Phys. Rev. Lett. 72 (1994) 1180-1183. (and Erratum Vol. 73 (1994) 3181.)
• R.S. Hayano et al., Phys. Rev. Lett., 73 (1994) 1485-1488.
• B. Ketzer et al., Phys. Rev. Lett. 78 (1997) 1671-1674.
• H.A. Torii et al., Nucl. Instr. and Methods in Phys. Research, A 396 (1997) 257-271.
• E. Widmann et al. Phys. Lett B, 404 (1997) 15-19.
• H.A. Torii et al., Phys. Rev A 59 (1999) 223-229.