We calculate the modification of a rho meson in nuclear matter through its coupling to resonance-hole states. Starting from a recently proposed model, we include all four star resonances up to 1.9 GeV. In contrast to previous works, we include not only resonances that couple to the rho in a relative p-wave, but also those that couple to an s-wave state. In addition, we solve the equation for the rho spectral function self-consistently. We find that s-wave resonances affect the in medium spectral function of the rho strongly. In the transverse channel the rho meson is, especially at non zero momentum, completely washed out and can in the presence of nuclear matter no longer be viewed as a resonant excitation of the vacuum. Instead, our model shows a continuum of possible excitations with the quantum numbers of a transversely polarized rho. In the longitudinal channel, however, the rho retains its resonant character in our calculation. As a consequence of the self-consistent treatment we also find a strong enhancement of the widths of the included nucleon resonances in medium. * Work supported by BMBF and GSI Darmstadt.

Using a simple parametrization of Breit-Wigner type for the hadronic side of the QCD sum rule for ρ mesons in vacuum as well as in a nuclear medium we explore the range of values for the mass and the width of the ρ meson which are compatible with the operator product expansion.

A relativistic and non-local model for the coherent photoproduction of pions on spin-zero nuclei is presented. The production operator is derived from an effective Lagrangian model that contains all the degrees of freedom known to contribute to the elementary process. Using the framework of DWIA, the matrix elements of this operator are evaluated using relativistic bound state wave functions. Final state interactions are accounted for via a pion-nucleus optical potential. The effects of in-medium modifications of the production operator are discussed. We give results for 12 C and 40 Ca, and compare our calculations to the data of the A2 collaboration at MAMI.

The coherent photoproduction of η-mesons on spin-zero nuclei is studied in a relativistic, non-local model, which we have previously applied to the coherent photoproduction of pions. We find that different off-shell extrapolations of the elementary production operator lead to large effects in the cross section. We also show that the almost complete suppression of the N (1535) seen in earlier studies on this reaction is a result of the local or factorization approximation used in these works. Non-local effects can lead to a considerable contribution from this resonance. The relative size of the N (1535) contribution depends on the structure of the nucleus under consideration. We give an estimate for the contribution of the N (1520) resonance and discuss the effect of an η-nucleus optical potential.

Abstract. We calculate the cross section for the production of q-mesons via AN---+NN~ in a relativistic OneBoson-Exchange-Model. Using this cross section we then determine the probability for the production of an qmeson by a A-resonance moving in nuclear matter. The result is compared to prescriptions in BUU-calculations in which q-production proceeds both through a direct channel and through the sequential process A-*Nn; nN ~ Nq. 13.60 cross sections used for the production processes involving resonances in the entrance channel; the BUU-calculations mentioned just use an ad-hoc recipe for the direct reaction channel AN ~ NN meson and contain an explicit treatment of the sequential channel A --* Nn; nN --* Nq. PACS:It is thus the aim of this study to investigate the elementary production process in more detail in order to put the results on the dominance of resonance channels on a firmer ground. For this study we chose the process AN ~ NNq because all the interaction vertices occurring there are known from other processes.