In-medium properties of mesons

In the project B.4, the modification of meson properties (mass, width) in a nuclear medium has been studied in photoproduction of mesons off nuclear targets. This work has been motivated by theoretical expectations of in-medium modifications of hadrons based on the conjecture of a partial restoration of chiral symmetry in a strongly interacting medium. It has been shown that these in-medium changes can be discussed in a compact form in terms of an optical potential describing the meson-nucleus interaction. Experimental approaches to determine the real and imaginary part of the meson-nucleus potential have been developed. The experiments have been performed with the Crystal Barrel/TAPS detector at the electron accelerator ELSA (Bonn) and the Crystal Ball/TAPS detector at MAMI (Mainz). Measuring the excitation function and momentum distribution for photo production of. and eta' mesons, the real parts of the omega and eta'-nucleus potential, given by the in-medium mass shift, have been determined. For the eta' meson a lowering of the mass at normal nuclear matter density by -(39 +/- 7(stat)+/- 15(syst)) MeV is observed, while for the omega meson a slightly smaller mass shift is found, however, with much larger uncertainties, not excluding a zero mass shift. The imaginary part of the potentials has been extracted from the measurement of the transparency ratio which compares the meson production cross section per nucleon within a nucleus to the production cross section off the free proton. For the eta' meson the imaginary part of the potential is found to be smaller than the real part. In case of the omega meson the opposite is observed. This makes the eta' meson a good candidate for the search for meson-nucleus bound states while no resolved omega mesic states can be expected. The results are compared with theoretical predictions. An outlook on future experiments is given.