Symmetries and hadron dynamics go on the MENU

Hadron physics investigates one of the open frontiers of the Standard Model: the strong interaction for large gauge couplings. Experimentally, there are currently two major strategies. Precision experiments study symmetries and their violations with the aim of extracting fundamental quantities of QCD, such as the quark masses. Studies of the excited states and their decays, on the other hand, try to establish the ordering principles of the hadronic spectra to shed light on the problem of the confinement of the quarks.

The common aspects in both the charmed sector and the light quark sector were the major reason to bring together 350 experts from high-energy physics and nuclear physics to the 11th International Conference on Meson–Nucleon Physics and the Structure of the Nucleon (MENU 2007), which took place on 10–14 September 2007 at the Research Centre Jülich. The plenary sessions provided a broad review of the field, while invited and contributing speakers covered special topics, such as spin physics, meson and baryon spectroscopy, lattice calculations and in-medium physics, in five parallel sessions.

The light quark sector

Jürg Gasser, of the University of Bern, opened the conference with a review talk on chiral effective field theory, the standard tool for hadron physics in the threshold region. Lattice calculations have come into contact with chiral perturbation theory (_χPT) by obtaining values for the low-energy constants l₃ and l₄. The DIRAC and NA48 experiments at CERN have tested the predictions of χPT by studying the level shifts of pionium and the decay of charged kaons into three pions. Rainer Wanke, of Mainz University, reported on the recent high-statistics data from NA48/2. The data have allowed the extraction of the S-wave pion–pion scattering length with great precision from studies of the Wigner cusp in the two-pion subsystem, as Ulf-G Meissner and collaborators predicted in 1997. The results agreed with the χPT predictions after inclusion of isospin-breaking effects.

Johan Bijnens of Lund University emphasized in his review on η physics that the decays of both the η and the η’ mesons are good laboratories to study non-dominant strong interaction effects. The slope parameter α in the neutral three pion decay of the η is a puzzling challenge, as _χPT does not explain the sign of the slope parameter, even when pushed to next-to-next-to-leading order, while non-perturbative approaches do. Magnus Wolke of the Research Centre Jülich showed Dalitz plots for the decay of the η into three neutral pions, which the WASA-at-COSY Collaboration obtained in the first production run in April/May 2007. The WASA detector was transferred from Uppsala to Jülich in 2005. Cesare Bini of the Sapienza Università di Roma reviewed recent KLOE results featuring the η mass, measurement of η–η’ mixing, the slope parameter of the η decay and results on the scalar mesons ƒ₀(980) and α₀ (980) seen in φ-decay. Patrick Achenbach of Mainz showed the first results on η’ decays into η and two neutral pions from the CB-TAPS experiment at the MAMI-C electron accelerator at Mainz. Catalina Curceanu presented the recent progress on kaonic hydrogen by the SIDDHARTA collaboration at the DAΦNE facility, which will allow physicists to obtain the antikaon–nucleon scattering lengths.

Effective field theory is beginning to make an impact on traditional nuclear physics with a consistent treatment of two-nucleon and three-nucleon interactions. Theorists have for many decades considered three-nucleon forces as a possible explanation for the unsolved problem of the saturation properties of nuclear matter. Kimiko Sekiguchi of RIKEN, Stanislaw Kistryn of the Jagiellonian University Krakow, and Daniel Phillips of Ohio University showed how the possibilities of studying polarized proton–deuteron reactions provide a direct experimental access to the three-nucleon force. In addition, the progress in applying lattice methods to study hadrons, hadron–hadron interactions and eventually nuclei, figured in the talks by Silas R Beane of the University of New Hampshire, Uwe-Jens Wiese of the University of Bern, and Andreas Schäfer of the University of Regensburg.

The charm sector

The decay of heavy mesons produced by the present generation of electron–positron colliders sheds new light on the light meson sector because the scalar mesons ƒ₀(980) and α₀(980) are found in the decay products, for example in the reaction D⁰ → K⁰π⁺π^–, as Michael Pennington of Durham University stressed in his talk. Joseph Schechter of Syracuse University presented effective Lagrangian methods for the light scalar meson sector. The new charmed mesons D_s(2317) and D_s(2460), together with the charmonia-like states X, Y and Z, can be considered as unexpected contributions from B-factories, as Ruslan Chistov from ITEP Moscow pointed out in his overview of results from the Belle experiment at KEK. B-decays suppress the background contributions and offer large branching fractions, thus allowing an angular analysis to obtain quantum numbers. Walter Toki of Colorado State University discussed the recent results on the X(3872) and Y(3940) mesons from the BaBar experiment at SLAC and on the Z(4430) discovered by Belle, which apparently do not fit into the conventional quark–antiquark model for mesons. The Z(4430) may be a hadronic molecule made of a D*(2010) and a D₁(2420) or a tetraquark, and, if confirmed, would be as exciting as the first charged hidden-charm state.

Matthias Lutz from Gesellschaft für Schwerionenphysik, Darmstadt, and Craig Roberts of Argonne discussed various aspects of hadron spectroscopy, while Ulrich Mosel of the University of Giessen highlighted recent theoretical progress in modelling the medium-dependence of nucleon resonances. Ulrike Thoma of the University of Bonn reported on evidence for two new Baryons – a D₁₅(2070) and D₃₃(1940), seen in η production on the nucleon at the ELSA facility at Bonn. Bing-Song Zou, of the Chinese Academy of Science, Beijing, observed that J/ψ decay is an ideal isospin filter for studying baryons, allowing the identification of the elusive Roper resonance as a visible bump, quite in contrast to pion–nucleon scattering. The Roper resonance is the first excited state of the nucleon with the quantum numbers of the nucleon. Results from the Beijing Spectrometer experiment show a surprisingly small Roper mass of 1360 MeV.

Haiyan Gao of Duke University showed how quark–hadron duality studies in charged pion photoproduction can be used to obtain information about resonances in the energy region above 2 GeV. Kai Brinkmann of Technical University Dresden reviewed results from the cooler synchrotron COSY at Research Centre Jülich, in particular the negative result for the search for pentaquarks, while Takashi Nakano reported on the recent status of experiments at the SPring-8 synchrotron radiation facility in Japan. Mikail Voloshin, of Minnesota, reviewed the decay of charmed hadrons and pointed out the open opportunities to improve our knowledge of the Kobayashi–Maskawa matrix element V_ub. Ikaros Bigi, of Notre Dame du Lac, focused on D⁰ oscillations, which open a unique window on flavour dynamics.

The future will see exciting new machine developments. Naohito Saito discussed progress at the new Japan Proton Accelerator Research Complex, while the European project for the Facility for Antiproton and Ion Research was covered by Paolo Lenisa from the Università di Ferrara, Mauro Anselmino of INFN Torino and Johan Messchendorp of KVI Groningen. Anthony Thomas presented the 12 GeV upgrade for the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab, and Günther Rosner of the University of Glasgow gave an overview of physics with the CEBAF Large Acceptance Spectrometer.

Willem Van Oers of Manitoba University gave a lively address as the representative of the International Union of Pure and Applied Physics, who together with Forschungszentrum Jülich, Deutsche Forschungsgemeinschaft, Jefferson Lab, and the Hadron Physics I3 FP6 European Community Programme made this conference possible.

• The next MENU conference will be held in two years’ time in Newport News, Virginia, in 2010.