Exotic atoms cast light on fundamental questions

1 November 2006

A workshop in Trento explored how experiments on exotic atoms, deeply bound kaonic states and antihydrogen provide a low-energy route to addressing fundamental physics.

As particle physics heads towards tera-electron-volt energies with the Large Hadron Collider, it may be surprising to find that not all valuable research requires hadron beams of the highest energy available. Indeed, the opposite can be true. Experiments on processes that involve hadrons at kilo-electron-volt or even electron-volt energies can address some unresolved questions in quantum chromodynamics (QCD), its associated symmetries such as chiral symmetry, and CPT-invariance. This quickly developing field, which connects atomic, nuclear and particle physics, as well as astrophysics, was the subject of an international workshop, Exotic Hadronic Atoms, Deeply Bound Kaonic Nuclear States and Antihydrogen: Present Results, Future Challenges, which was held at the European Centre for Theoretical Nuclear Physics and Related Areas (ECT*) in Trento on 19–24 June. The workshop brought together some 50 experts in exotic atoms and nuclei to assess the current experimental and theoretical status of the field, and identify the most relevant topics to be addressed in the future. The rich programme extended from the pionic, kaonic and antiprotonic varieties of exotic atoms to antihydrogen, and exotic nuclear clusters, better known today as deeply bound kaonic nuclei. The workshop discussed the latest results from many experiments on these exotic atoms, and outlined future plans that are based on improved experimental techniques in detectors and/or hadronic beams.


Studies of exotic atoms in which a hadron such as a π, K or pbar replaces an electron can reveal important information about spontaneous chiral-symmetry breaking in QCD, which governs the low-energy interactions of the lightest pseudoscalar mesons (pions and kaons) with nucleons. Such experiments can access hadronic scattering lengths at zero energy by direct measurements of bound-state parameters, which is not possible through other experimental approaches. The Paul Scherrer Institut in Villigen has investigated pionic hydrogen (π–p) and deuterium (π–d), and DAFNE in Frascati has investigated their kaonic counterparts. Other no-less-important species include kaonic and antiprotonic helium, which have been studied at the Japanese High Energy Accelerator Research Organization (KEK) and CERN, and yet another exotic variety is formed by the non-baryonic π+– π(pionium) and πK atoms. Finally, the antihydrogen atom, pbar–e+, which CERN has copiously produced, is in a class of its own owing to its importance for testing the CPT theorem to extremely high precision.

The latest kaonic hydrogen results from the DAFNE Exotic Atoms Research (DEAR) experiment, as presented by Johann Marton of SMI Vienna, gave rise to a lively discussion on the possibility of accommodating them with kaon–nucleon scattering data. More precise data and further theoretical calculations will evidently be needed in this domain. The SIDDHARTA experiment that is planned for DAFNE, which aims at 10 times higher precision on the kaonic hydrogen atom and the first measurement on kaonic deuterium, should contribute to a better understanding of the physics of the kaon–nucleon interaction at very low energies.

In the pion–pion scattering sector, the DIRAC experiment at CERN has measured pionium and yielded new values for the scattering lengths. However, the study of the kaon decaying to three pions provides a valid alternative for determining these quantities to an unprecedented accuracy. Gianmaria Collazuol, of INFN and Pisa, presented results from the NA48/2 experiment at CERN on the (a0–a2) difference of pion–pion scattering lengths with a precision of about 6%, equally shared between systematic and statistical uncertainties. This value is in agreement with various theoretical predictions that were discussed at the workshop. Further refinements in the precision of the results will need an interplay between experiment and theory, since most of the systematic error is caused by theoretical uncertainties.


Researchers at the Antiproton Decelerator (AD) at CERN are pursuing precision spectroscopy of antiprotonic helium, as Ryugo Hayano of Tokyo described. The study of the metastable three-body system pbar–e–He2+ has led to the most stringent tests of the equality of the charge and mass of the proton and antiproton at a relative precision of 2 ppb and, for the first time, produced a value of the antiproton-to-electron mass ratio.

Antihydrogen, the simplest atom of antimatter, offers even higher sensitivity to violations of CPT symmetry, because the properties of its conjugate system, hydrogen, are known to very high precision (10-12 for the ground-state hyperfine structure, and a few parts in 1014 for the 1S–2S two-photon transition). The ATRAP, ALPHA and ASACUSA collaborations at the AD are pursuing the formation of cold antihydrogen atoms for precision spectroscopy. Nikolaos Mavromatos of King’s College London and Ralf Lehnert of Massachusetts Institute of Technology discussed theoretical aspects of these interesting tests of CPT and Lorentz invariance, as well as the possibility of using antihydrogen to investigate the gravitational properties of antimatter.

Finally, an important part of the workshop programme discussed a new type of exotic nucleus – the antikaon (Kbar)-mediated bound nuclear system, with Kbar as constituents. Soon after the theoretical prediction of such states, both KEK and the Laboratori Nazionali di Frascati reported preliminary experimental evidence for their existence, and several experiments in Europe and Japan are currently searching for them. Attendees heard a critical review of the present experimental results, followed by an extended discussion on the foundations of the predictions. A lively discussion took place between those defending the existence of these deeply bound kaonic nuclear states and others who express doubts on their existence. There was also a critical analysis of the conditions under which such states might exist. New experiments, studying both the formation and the decay processes of the exotic nuclei, will play a key role in clarifying this interesting physics. These include a recently accepted proposal at the Japanese Proton Accelerator Research Complex (J-PARC), the AMADEUS experiment at DAFNE and the future possibility of investigating double Kbar systems at the Facility for Low-energy Antiproton and Ion Research (FLAIR) within the international Facility for Antiproton and Ion Research (FAIR) to be built at Darmstadt.

A round table on the deeply bound Kbar–nuclear systems, led by Wolfram Weise of Munich, concluded the workshop. He stressed the importance of new experimental studies and further theoretical efforts, showing that progress is expected on one side from next-generation experiments, and on the other by understanding of the Kbar–nucleon interaction (in the SIDDHARTA experiment) by realistic modelling of the nucleon–nucleon interaction and of the Kbar–nucleon–nucleon→hyperon–nucleon absorption. Weise also discussed the interesting connection with dense matter, namely kaon condensation in high-density media.

The workshop confirmed that many fundamental and still-open questions in low-energy QCD and related symmetries can be assessed and answered with experiments in the low-energy domain, by creating and measuring new forms of matter – exotic atoms, exotic nuclei and antihydrogen. An active and growing scientific community supports the great expectations of the field.

• For the full programme and the compete list of speakers see

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