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Mesons in the medium - experimental probes for chiral symmetry restoration?

The in-medium modifications of hadron properties have been identified as one of the key problems in understanding the non-perturbative sector of QCD. Several theoretical papers discuss the possibility of a partial restoration of chiral symmetry in a strongly interacting environment. However, is it possible to find experimental evidence for partial symmetry restoration by studying the in-medium behaviour of mesons, and the meson-nucleus interaction. Is this interaction sufficiently strong to allow even the formation of mesic states only bound by the strong interaction? The answers can be given by studying the meson-nucleus complex potential. The experimental approaches to deduce this potential and the experimental results from CBELSA/TAPS are presented and discussed. Data taken on a C and Nb target at the Crystal Barrel (CB)/TAPS detector system at the ELSA accelerator facility in Bonn have been analyzed to deduce the η'- and ω-nucleus potential. The data for both mesons are consistent with a weakly attractive potential. The formation and population of ω-nucleus and η'-nucleus bound states is discussed. In case of the ω meson the in-medium width is found to be larger than the potential depth which hampers a successful experimental identification of ω-mesic states. The relatively small in-medium width of the η' meson encourages ongoing experiments to search for η'-nucleus bound states.