Aschaffenburg, a medieval town near Frankfurt in Germany, was the attractive setting for the 10th International Conference on Hadron Spectroscopy, held from 31 August to 6 September. The reason the meeting was held in Germany for the first time in its history was the decision of GSI in Darmstadt to include hadronic physics in future research by constructing an antiproton source and a storage ring (HESR/PANDA) for antiprotons, thus extending the programme that was so successfully started with LEAR at CERN.
The conference, jointly organized by GSI and the Institute for Experimental Physics of the Ruhr-Universität Bochum, was attended by around 200 scientists from all over the world. The timing of the conference was very fortunate as many new and surprising results appeared in the months just prior to the meeting.
The highlights of this year’s conference were the discussions about the nature of the recently discovered narrow states, the widths of which are compatible with the experimental mass resolutions, of the order of 10 MeV.
In the mesonic sector, new results came from BaBar, Belle, CLEO and BESII. Two narrow states with masses of 2317 and 2460 MeV with open charm and strangeness have been seen, and may be the missing 0+ and 1+ states. Their masses and widths are, however, difficult to explain in standard quark model calculations, and so explanations in terms of D-K “molecules”, D-π “atoms” and charmed four-quark states are under intense discussion. The observed pattern fits very well to chiral model predictions, giving additional weight to these ideas.
Belle has found another very narrow (Γ < 3.5 MeV) state at 3872 MeV decaying into J/Ψπsup>+π–. Its properties do not fit the values expected for the still missing 1 3D2 ccbar state well, therefore an explanation in terms of a D0D0bar* molecule is not excluded. BESII has reported a bump in the ppbar mass distribution, which might belong to a ppbar bound state below the ppbar threshold. The interpretation of this structure, however, is still controversial.
As far as baryons are concerned, there is evidence from four different laboratories for the existence of a narrow state, Θ+ with a mass of 1540 MeV decaying to nK+. Having positive strangeness, it is a very exotic particle and cannot be constructed in a conventional three-quark picture. All signals have a significance of five standard deviations and their confirmation continues. Such a state was predicted six years ago using a soliton model giving rise to an antidecuplet with JP = 1/2+. The implications of these findings were discussed in several talks at the conference and in an open panel, resulting in many ideas for future measurements that will clarify the true nature of these states.
Additionally, there were some very interesting contributions concerning the properties of hadrons inside nuclear matter, the discovery of baryons with double charm and its implications, and the role of the sigma/kappa structures in low-energy ππ and πK scattering. Although new results concerning the sigma/ kappa problem were presented, it is not yet clear if these structures can be attributed to particles or are effects of dynamical origin.
One outcome of the conference is that it has become very clear that the advent of precision data in the heavy quark sector is of high relevance for future developments in hadron physics, even in the light quark sector. It seems that the quark-antiquark and three-quark descriptions of hadrons have reached their limitations and have to be extended or replaced by new ideas, such as chiral models, soliton pictures, molecular states, and so on, which have to be taken more seriously than in the past.
The proceedings of HADRON 2003 will be dedicated to Lucien Montanet from CERN, who died on 19 June this year. He belonged to the pioneers of hadron physics and his eminent role in this field was highlighted during a special plenary session in his honour. The next conference in the series is scheduled to take place two years from now in Rio de Janeiro, where there should be further exciting results. Undoubtedly, hadron physics has a bright future.
