Modification of pion pairs in nuclear matter

In this work the quasifree photoproduction of pion pairs on bound nucleons is analyzed for the solid targets carbon, calcium and lead. These measurements allow an isospin dependent determination of the in medium properties of pion pairs. The invariant mass distributions showed a shift in strength towards the threshold region with increasing atomic number of the targets in case of an isoscalar ππ meson system. According to theoretical predictions this effect was interpreted as a positive signature of a partial restoration of chiral symmetry. A pioneering experiment with photon beams was performed by the TAPS collaboration at the accelerator facility MAMI-B in Mainz in 1995. This measurement observed an invariant mass shift of the isoscalar π0π0 channel with increasing atomic number. In the π±π0 channel this effect was not observed. This observation confirmed the former experiments, since final state interactions of the pions would affect in first order both isospin channels in a similar way. Due to the poor statistics, the significance of the data was however limited. The experiment described in this work reached a much higher statistical significance, allowing a review of the old data and a study of the dependence of the signal from parameters like the momenta of the ππ system or the single pions. In this experiment, the TAPS detector was used as a forward wall in combination with the Crystal Ball detector to achieve a 4π solid angle coverage for photon detection at the MAMI accelerator facility. The new readout electronics for the BaF2 crystals was used for the first time. The new data analysis confirms the previously measured and published data. However, the shift of the invariant π0π0 mass to the threshold region is less pronounced than in the published data. By performing cuts on the individual pion momenta, resulting in a maximum mean free path of the pions, the shift towards lower masses is enhanced. BUU transport calculations can not describe the different behavior in the isospin channels, which may partly be due to uncertainties in the production crossection on the neutron. Therefore, a final interpretation of the observed mass shift - whether it can be described by final state interaction alone or by a partial restoration of chiral symmetry - can not be made as yet. With the cross section determination on the neutron, done by Stefan Lugert in his ongoing PhD work, the BUU calculations can be refined and compared to the analyzed data. Another part of my PhD thesis has been the development of the new data acquisition, based on the parallel readout of the newly developed readout electronics. The 8 VME crates, containing 16 readout modules each, were read out by one VME computer in every crate. The data was sent to a master PC, which combined the data packets to a global event. This event can be stored on disk e.g. when TAPS is operated in stand-alone mode, or it can be sent to the data acquisition of another detector system. During our measurement campaign, the Acqu system, that reads out the tagger and the Crystal Ball, was used as the master system. A major issue was to guarantee the synchronisation of all detector sub-events. (orig.)