Siegfried Krewald reports the news from the Hadron Physics at COSY workshop.
On 7-10 July this year around 120 physicists convened at the Physikzentrum, at Bad Honnef in Germany, to attend the Hadron Physics at COSY workshop. Experimenters and theorists discussed the key questions that can be addressed with cooled beams of polarized and unpolarized protons and deuterons, in particular at the cooler synchrotron, COSY, at the Forschungszentrum Jülich.
The workshop began with an overview of charge symmetry breaking (CSB) by Gerald Miller of Seattle. If the up and down quarks had the same mass, quantum chromodynamics would be invariant under the exchange of up and down quarks. However, such charge symmetry is broken by electromagnetic effects and the mass difference of the up and down quark. This makes the neutron heavier than the proton, which is essential for the stability of the hydrogen atom. Investigating CSB effects is an important way to determine the masses of the up and down quark, two fundamental parameters of the Standard Model. On the theoretical side, Weinberg’s concept of effective field theories has provided an important tool, enabling precision calculations of CSB to be made.
Alena Opper of Ohio reported on the recent experimental breakthroughs in identifying CSB in the reactions np → dπ0 and dd → απ0. CSB is also being investigated in pion production at COSY. A photon detector would create the unique possibility of studying the mixing of the two lightest scalar mesons, the isoscalar f0(980) and the isovector a0(980), which is induced by CSB. The reactions pn → dπη and dd → απη promise to be particularly clean tools for independently quantifying the a0 – f0 mixing model, to help resolve the controversy as to whether or not these systems have a quark-antiquark structure.
The hyperon-nucleon interaction is an ideal testing ground for studying the breaking of another symmetry, SU(3)flavour, in hadronic systems. So far, relatively few data obtained in the 1960s and recently at KEK are available on hyperon-nucleon scattering at low energies, so production experiments must be used instead. Ben Gibson of Los Alamos and Ashot Gasparyan of ITEP reported on theories for K–d → γYN and pp → K+YN. They showed how to isolate final-state interaction effects by studying polarization observables and how to extract independently the spin dependence of the hyperon-nucleon scattering length model. This is relevant for understanding hypernuclei and the structure of strange matter.
In the near future, COSY will provide precise data on meson production in reactions involving polarized baryons. These data will be both a challenge and an opportunity for effective field theories. New data on η, ω and φ meson production were presented in separate sessions. In addition, the production of scalar mesons is of special interest because of the possible mixing with glueballs, as Eberhard Klempt of Bonn explained.
Baryon resonances are an important part of the research programmes of many accelerator facilities for hadron physics. Maxim Polyakov of Bochum presented his predictions for the pentaquark, Θ+, a baryon that does not fit into the standard three-valence quark structure (see “New five-quark states found at CERN”). Wolfgang Eyrich of Erlangen reported on hyperon production in proton-proton reactions and pointed out the possibilities for confirming the existence of the Θ+ with hadronic reactions. Another candidate for “exotic” structure, the Roper resonance N*(1440), is also under intense experimental investigation. Polarized electrons will be used at the electron-scattering facility MAMI in Mainz to discriminate the Roper contribution from the background, while the experiment CLAS at JLAB uses electroproduction to study isobars as a function of the photon virtuality. The COSY facility offers polarized protons and deuterons as incident particles, as well as the alpha particle in inverse kinematics. Here, the alpha particle can be used as a scalar-isoscalar probe.
Hadronic interactions are also of fundamental interest for the spontaneous breakdown of chiral symmetry, as described in the review by Volker Metag of Giessen. The current masses of the up and down quarks are only a few per cent of the proton mass, indicating that the bulk of hadronic mass is due not to the Higgs mechanism but to the spontaneous breakdown of chiral symmetry. An important question is whether chiral symmetry is restored at high nuclear densities. Studies of deeply bound pionic atoms and two-pion production on nuclei are viable tools for exploring these issues.
The extension of the GSI laboratory at Darmstadt, approved earlier this year, offers the prospect of studying hadron physics with antiprotons at energies up to 15 GeV. Hans Gutbrod of GSI presented the planned new research facilities, while Bernhard Franzke, also from GSI, outlined the design of the High Energy Storage Ring (HESR). Helmut Koch of Bochum then introduced the highlights of the charm-physics programme at HESR, and the role of charm in the nuclear medium was addressed by Jim Ritman of Giessen.
