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Spectroscopy of free and bound baryon resonances in pion-induced reactions


Research objectives and content
A major goal of the current research programme is to identify effect of the chiral simmetry restoration. While broken for strongly interacting free particles, chiral simmetry is predicted to be restored with increasing nuclear density and temperature, resulting in a change of the hadron masses as compared to their free values. It is proposed to perfom Ames spectroscopy of the free and bound N(1535) nucleon resonance following pion-induced reactions on targets of liquid hydrogen as well as on solid targets of complex nuclei exploiting the secondary high-momentum pion beams available at pion beam facility of GSI Darmstadt. Mass reductions of about 20% can be expected. The n meson is identified via an invariant mass analysis of the two decay photons corresponding to the 40% decay channel of the meson. The photons are observed in a large modularized two-arm photon spectrometer which represents one of the main experimental tool to be t in the proposed experiments.
Two tasks have to be perfomed before the actual measurements. One is the careful study of the experimental arrangement using extended computer simulations. Here the response of the detectors to the two photon decay of the , signal is taken i account and the effects of electronic triggers that have to be implemented are investigated in order to enhance the n sign against the background of unwanted reactions on the event level. These studies will require the first 5 months of the proje The second task is to install the TAPS photon spectrometer in its optimized geometry at the pion beam line. Commissioni runs of the pion beam are required in this place as well as tests of the new TAPS electronics which is now under developn by the collaboration. The result will be a complete description of the experimental set up to be use. This part should be rea on January 1998. The analysis of the data is complex and time consuming. Within a period of 6 months a preliminary ana of the data can be completed, resulting in the primary physics results of the measurements. A complete analysis will requi 5 months more.
Training content (objective, benefit and expected impact)
The spectroscopy of the bound N(1535) nucleon resonance produced in pion-induced reactions will provide a crucial test the theoretical predicitons that observalble precursor phenomena of chiral simmetry restoration already occur at normal nuclear matter density. Mass reductions of about 20% as compared to the free value can be expected. If substantiated such effects would clearly demonstrate the ability of present QCD-based model calculations to described nuclear matter in the pertubative-region .

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EU contribution
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