Meson Production in Photonuclear, π-induced and Heavy-Ion Reactions: Elementary Processes and Medium Effects

The structure of the nucleon has been investigated by studying the photoexcitation of nucleon resonances and their subsequent meson decay. The photon spectrometer TAPS has been used in a series of corresponding experiments at the Mainz microtron MAMI. A comparison of the photoproduction of mesons on the free nucleon and on nucleons bound in nuclei reveals differences attributed to changes in the properties of hadrons in the nuclear medium. Most of the nucleon resonances exhibit only a small shift in mass but an appreciable broadening. The largest effect is observed for the D13-resonance which may be attributed to its ρ-meson decay. Various calculations predict a change in the spectral function of this vector meson in the nuclear medium. First experimental evidence for medium modifications of vector mesons may have been observed in ultra-relativistic heavy-ion collisions, e.g. with the CERES detector. A dedicated program focusing on the properties of mesons in the nuclear environment at different temperatures and baryon densities will be taken up with the HADES di-lepton spectrometer at GSI, utilizing the π- and heavy-ion beams from SIS. Photonuclear, hadron and heavy-ion induced reactions are thus complementary approaches to study the properties of hadrons in nuclear matter. (author)