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Zen and the art of low-energy antiproton experiments

2 November 1999

A recent workshop in Japan set the scene for a range of experiments at CERN’s AD machine, which will synthesize and explore atoms of antimatter. John Eades reports.

As CERN’s Antiproton Decelerator comes into operation a new era begins for high-precision studies of antiprotonic atoms and antihydrogen, as well as for the antiproton itself and the way in which it interacts with ordinary atoms.

The imminent arrival of these high-quality antiproton beams of extremely low energy (5 MeV) was heralded at the International Workshop on Atomic Collisions and Spectroscopy with Slow Antiprotons, held in Tsurumi on 19-21 July.

The workshop included a Zen Buddhism course (Zazenkai). Perhaps for the first time in a physics workshop, jet-lagged attendees were woken up at 3.30 a.m. for meditation practice in the temple.

Progress on the AD programme

In the more conventional sessions, 55 participants from 25 institutions in Europe, Japan, Russia and the US discussed theoretical, technical and experimental progress on the Antiproton Decelerator (AD) experimental programme. At the moment it follows two tracks. One is the ASACUSA collaboration’s programme of antiproton­atom collisions and laser/microwave spectroscopy of antiprotonic helium (in which one of the two normal orbital electrons is replaced by an antiproton). The other track is the synthesis and spectroscopic study of antihydrogen atoms by the ATHENA and ATRAP collaborations.

One motivating force behind ASACUSA is that, in much the same way as the hydrogen atom spectrum revealed the properties of the proton and electron over the course of the 20th century, high-precision laser and microwave measurements of atomic transitions of the antiproton in antiprotonic helium can reveal, with commensurate precision, the properties of the antiproton itself. Such measurements constitute valuable tests of validity of underlying physics symmetries. Antiprotonic helium was chosen instead of the simpler two-body protonium (antiprotonic hydrogen ­ a proton and an antiproton in orbit round each other) atom because it is stable against annihilation for some microseconds after formation, while protonium, under normal conditions, is not.

ASACUSA experiments that are already approved include a microwave triple resonance experiment on hyperfine splitting caused by the interaction of the electron spin and antiproton orbital moments, and a search for laser-induced transitions between, so-far unobserved, atomic levels.

In addition, a new high-resolution laser system is expected to reduce the measurement precision of all transition frequencies to less than one part per million ­ the level at which quantum electrodynamic effects appear. This has already been achieved in experiments that were studying another transition at CERN’s LEAR low-energy antiproton ring, which closed in 1996.

The status of ASACUSA on these fronts was reported by K Komaki and M Hori (Tokyo). The interpretation of spectral features of antiprotonic helium in terms of the properties of the atomic constituents requires energy-level calculations with precision similar to that of the experimental values, and this was discussed by Y Kino (Sendai), D Bakalov (Sofia), V I Korobov (Dubna) and G Korenman (Moscow).

Another goal of ASACUSA is to extend to lower energies the Aarhus and Tokyo LEAR experiments on the atomic interactions of antiprotons. This has sparked considerable theoretical interest. Contributions came from H Knudsen (Aarhus), P Krstic (Oak Ridge) and A Igarashi (Miyazaki) on ionization and energy loss for antiprotons interacting with matter. Such experiments require 10­100 keV rather than 5 MeV antiprotons. They will be produced by inserting a decelerating radiofrequency quadrupole (RFQ) in the AD beam.

This is under construction in CERN’s Proton Synchrotron division and will soon be tested in Aarhus. Antiprotons from the RFQ may be used directly or, for certain ASACUSA experiments, collected and cooled in a multiring harmonic trap (T Itchioka, Tokyo), where they will be reaccelerated to electron-volt or kilo-electron-volt energies.

Traps for charged (as well as neutral) particles feature prominently in the plans of the ATHENA and ATRAP collaborations. Here the main aim is to use the laser probes to compare identical atomic transition frequencies in hydrogen and antihydrogen.

Such experiments have an important advantage over those using antiprotonic helium. They are direct comparisons, in which symmetry-conjugate systems are compared without any need for theoretical input. On the other hand, the technical problems associated with producing antihydrogen are much more complex than is the case for antiprotonic helium, which is created in abundance whenever antiprotons are slowed to electron-volt energies in helium gas. In particular, the antihydrogen must be synthesized at micro-electron-volt rather than electron-volt energies.

For this, AD antiprotons and positrons from radioactive sources must first be collected as plasmas in suitable containers and cooled to liquid helium temperatures. M Holzscheiter (Los Alamos) reported on the progress of the ATHENA experiment, which aims to synthesize antihydrogen atoms at sub-Kelvin temperatures. K Fine (CERN) and H Totsuji (Okayama) discussed the behaviour of these plasmas in Penning traps (with hyperboloidal electrodes) and in Penning-Malmberg traps (cylindrical ones), both of which are adequate as plasma bottles for these processes.

Future proposals

As befits the promise offered by the birth of a new machine, many ideas that go beyond the current AD programme were presented in Tsurumi. They include the possibility that atomic protonium, so far ignored because of its short lifetime, may live long enough under near-vacuum conditions to be the subject of ASACUSA-type experiments (R S Hayano, Tokyo). Another possibility is the existence of metastable antiprotonic lithium (K Ohtsuki, Chofu-shi) and of antiprotons in solution in liquid helium (T Azuma, Tsukuba). H Schmidt­Böcking (Frankfurt) presented a proposal for a table-sized antiproton storage ring.

The Tsurumi workshop was organized by Yasunori Yamazaki of the Tokyo University Komaba campus and RIKEN, and it was sponsored by the Antimatter Science Project of the University of Tokyo, the Danish Natural Science Research Council’s Centre for CERN-related Atomic and Nuclear Physics and the Japanese RIKEN (Rikagaku Kenkyuujo) Institute.

In his concluding remarks, Mitio Inokuti (Argonne) commented on the confidence and excitement with which this worldwide physics community awaits AD beams. Tantalizing hints of this physics were revealed during the era of LEAR, of which the AD is now a worthy successor.

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