The fascinating world of strange exotic atoms

20 January 2010

A workshop addresses the renaissance in the field of hadronic atoms and nuclei.


The field of exotic atoms has a long history and it is currently experiencing a renaissance, from both the experimental and theoretical points of view. On the experimental side, new hadronic beams are either already available, with kaons at the DAΦNE facility at Frascati, or will soon become available with the start-up of the Japan Proton Accelerator Research Complex (J-PARC). New detectors, with improved performance in energy resolution, stability, efficiency, trigger capability etc, are also starting to operate. On the theoretical side the field has advanced significantly through recent developments in chiral effective-field theories and their applications to hadron–nuclear systems. In light of these developments it was appropriate for the international workshop “Hadronic atoms and nuclei – solved puzzles, open problems and future challenges in theory and experiment” to address these topics on 12–16 October 2009, at the European Centre for Theoretical Studies in Nuclear Physics and related areas, ECT*, Trento.

Unique methods

So what are hadronic atoms and why is there a growing interest in studying them? An exotic hadronic atom is formed whenever a hadron (pion, kaon, antiproton) from a beam enters a target, is stopped inside and replaces an orbiting electron. Such an exotic atom is usually formed in a highly excited state; a process of de-excitation through the respective atomic levels then follows. The X-ray transitions to the lowest orbits (1s) are affected by the presence of the strong interaction between the nucleus and the hadron, which shifts the 1s level with respect to the value calculated on a purely electromagnetic basis and limits the lifetime (increases the width) of the level. Extracting these quantities via the measurement of the X-ray transitions provides fundamental information on the low-energy hadron–hadron and hadron–nuclear interactions, which is impossible to obtain by any other method. Quantities such as kaon–nucleon scattering lengths, for example, turn out to be directly accessible by measuring the properties of exotic atoms. These are key quantities for dealing in a unique way with important aspects of low-energy QCD in the strangeness sector, such as chiral-symmetry breaking.

The DAΦNE Exotic Atoms Research (DEAR) experiment has measured kaonic hydrogen with unprecedented precision, which led to a lively debate at the workshop on the procedure for extracting the kaon–proton scattering length as well as its compatibility with existing kaon–nucleon scattering data. The SIDDHARTA collaboration, also at DAΦNE, presented the results of an even more precise measurement performed in 2009 on kaonic hydrogen, which will be complemented with an exploratory measurement of kaonic deuterium. The E570 experiment at KEK and SIDDHARTA have both measured kaonic helium and found that there is agreement with theory, thereby solving the “kaonic helium puzzle” – a long-standing discrepancy between measured and theoretical values for the 2p level in 4He. The new E17 experiment planned at J-PARC will in the near future measure the X-ray spectrum of kaonic 3He with the highest precision. With other experiments already in the pipeline at existing and/or future machines at GSI, J-PARC and DAΦNE, the future of hadronic atoms will extend its horizons both in terms of precision as well as in dealing with new types of exotic atoms not previously measured, such as kaonic deuterium or sigmonic atoms (where a sigma replaces an electron).


Another hot topic that was intensively discussed at the workshop concerns the recent studies of K mediated bound nuclear systems. Theory originally suggested that the (strongly attractive) isospin I=0 KN interaction in few-body nuclear systems can favour the formation of discrete and narrow K-nuclear bound states with large binding energy (100 MeV or even more). However, recent work suggests that such deeply bound kaonic nuclear states do not exist: antikaon–nuclear systems might be only weakly bound and short-lived. There are different interpretations for the existing experimental results based, for example, on the interaction of negative kaons with two or more nucleons. This topic is related to a new puzzle in the physics of kaon–nucleon interactions: the nature of the Λ(1405) – does it have a single- or double-pole structure? There were long discussions about this at the workshop.

New frameworks

All of these topics have important consequences in astrophysics, for example, in the physics of neutron stars. The workshop reviews of experimental results covered experiments at KEK, Brookhaven and Dubna, as well as FINUDA at DAΦNE, the FOPI detector at GSI, OBELIX at the former Low-Energy Antiproton Ring at CERN, and the DISTO detector at the former Saturne laboratory in France. There was also a critical review of current theories and models. Discussions about future perspectives centred on an integrated strategy in which complementary facilities should bring together the various pieces of the overall puzzle. Among these are experiments proposed at J-PARC (E15, E17), GSI (upgrades of the FOPI and HADES detectors) and DAΦNE (AMADEUS), together with the possibility of using antiprotons to create single- and double-strangeness nuclei at CERN, J-PARC or the Facility for Low-energy Antiproton and Ion Research at GSI.

The workshop proved that the field of hadronic atoms and kaonic nuclei is active. While some puzzles, such as those concerning kaonic hydrogen and kaonic helium, are now solved thanks to the newer experiments (E570 at KEK, DEAR and SIDDHARTA at DAΦNE), many problems remain unresolved, or “open”. The workshop formulated and targeted important questions that still need experimental results and deeper theoretical understanding. There are many future challenges in both the experimental and theoretical sectors, which were formulated within a single framework for the first time.

There was also a round-table discussion, led by Avraham Gal from the Hebrew University of Jerusalem, that dealt with the search for the K-nuclear bound state. This proved extremely useful because it established common ground on what information (i.e. experimental results) could bring light to the field in future. This is important because new experiments are about to start, including the upgrades to AMADEUS, E15, HADES and FOPI.

The five-day workshop also included a visit to the Fondazione Bruno Kessler (FBK) centre for scientific and technological research. This gave the opportunity for the FBK to demonstrate its capacity to perform research in the field of frontier detectors for future experiments and to establish contacts with experiments that are potentially interested in such developments. In addition, there were presentations of the EU Seventh Framework Programmes (FP7), with Carlo Guaraldo of LNF-INFN Frascati describing the HadronPhysics2 project. In particular, experimentalists and theoreticians came together in a session dedicated to the LEANNIS Network in HadronPhysics2 FP7 – a network that focuses on low-energy antikaon–nucleon and nucleus interactions – in which topics and perspectives in the field were presented and discussed.

One important success of the workshop was that young people made up around half of the participants and that researchers from many countries took part, including Israel and Iran. This made it an occasion for not only scientific exchanges but for cultural and social ones as well, proving once again that scientists are part of society, with an important role.

bright-rec iop pub iop-science physcis connect