IKP

In charge: Prof. H. Ströher, IKP, h.stroeher@fz-juelich.de

Aims and Objectives

The objective of the FE-program is to investigate the inner structure and the interactions of the building blocks of matter ("particles"), which are comprised of quarks and gluons and thus are subject to the strong interaction, in order to obtain a comprehensive and quantita-tive understanding - one wants to understand how nature "makes hadrons". To achieve this goal, extensive experimental investigations with complementary probes (photons, electrons, hadrons, complex nuclei) as well as the development and building of models to describe the data in a consistent way, are necessary - based on the fundamental underly-ing interaction (Quantum-Chromodynamics, QCD). With the COSY-facility (accelerator, in-ternal and external detectors) at IKP of FZJ, a unique device is available to produce precision data for hadron-induced reactions – in particular polarization observables.

In the framework of hadron physics experiments at COSY the following important results have been achieved in the past year:

(1) The COSY-11 experimental facility has completed comparative experiments of protonproton induced lambda- and sigma-production. Kaon-Sigma production measurements were performed to determine the strength and phase relation of the different boson exchange contributions.

(2) At TOF kinematically complete hyperon production measurements have been taken with large angular acceptance. In the data analysis a hint was found for the exotic pentaquark state that had been recently discovered. From the Dalitz-plot of the reaction it can be inferred that the reaction happens predominantly via nucleon resonances.

(3) At GEM (BIG KARL) a precise determination of pion-spectra allowed to make conclusions concerning the proton-neutron interaction. Additionally, the mass of the eta-meson has been measured with unparalleled accuracy.

(4) At ANKE the production of eta-, omega- as well as K⁺ -mesons in proton-neutron reactions has been measured. The K⁺-production cross section is significantly larger compared to proton-proton collisions. In addition, the production of the scalar a₀-meson was investigated to shed light to its internal structure. The deuteron breakup in pd collisions – partly using polarized beams - and the charge exchange reaction pd - dp have been studied to obtain information on the nucleon-nucleon interaction.

Hadron physics experiments at external facilities contributed the following notable results:

(1) ATRAP at the CERN AD succeeded in the background free detection of anti-hydrogen and made first measurements of the atomic states and velocity distributions.

(2) The analysis of first phase measurements of pionic hydrogen atoms aimed at the PSI to determine the pion-nucleon scattering length is nearly completed.

Important results obtained in theoretical physics have been:

(1) A new chiral counting system has been developed for pion production in proton-proton collisions based on the momentum transfer, which allows a model independent analysis of the COSY-data.

(2) A new method to determine the hyperon-nucleon scattering length from production reactions has been developed, which is applicable to COSY experiments.

(3) The analysis of a newly discovered charm-strange meson in the framework of the Jülich meson-exchange model reveals a significant quark-anti-quark component.

(4) In the vicinity of the lambda (1405) resonance two different states are theoretically predicted.

(5) The production of dijets in deep-inelastic scattering on nuclei has been investigated.

(6) A theory to calculate the dipole excitation of halo-nuclei has been developed.