We present a calculation of e + e − production in γA reactions at MAMI and TJNAF energies within a semi-classical BUU transport model. Dilepton invariant mass spectra for γC, γCa and γPb are calculated at 0.8, 1.5 and 2.2 GeV. We focus on observable effects of medium modifications of the ρ and ω

We investigate the production of pions in heavy-ion collisions in the energy range of 1 -2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides the N (938) and the ∆(1232) we also take into account nucleon resonances up to masses of 1.9 GeV /c 2 as well as π-, η-and ρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (N N → N N ππ) on the pion spectra in comparison to π − data from Ar + KCl collisions at 1.8 GeV/A and π 0 -data for Au + Au at 1.0 GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data for Ar + KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reactions dynamics is well described by our novel transport approach.

The medium modifications of antikaons in dense matter are studied in a coupled channel calculation for scenarios more closely related to the environment encountered in heavy-ion collisions. We find that the optical potential of the antikaons turns repulsive or is drastically reduced for finite momenta or finite temperature. Hence, the antikaon mass does not decrease substantially in heavy-ion collisions to provide an explanation for the observed antikaon production rates at threshold. We demonstrate that the in-medium production cross section of antikaons via pions and Σ hyperons is remarkably enhanced up to an order of magnitude. The effect of in medium correction on the resulting kaon spectra is studied within a transport model. We find that both in medium correction lead to about the same enhancement of the spectrum. However, once the temperature dependence is taken into account no enhance- *

We calculate total photon-nucleus and electron-nucleus cross sections γA → X, eA → e ′ X and several cross sections on pion and eta production in nuclei in the energy regime between the first and the third resonance region for photon virtualities Q 2 ≤ 1.0 GeV 2 within a semi-classical BUU transport model. In both cases we discuss the varying influence of several medium modifications on the cross sections for different values of Q 2 .

We calculate the total photoabsorption cross section on nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical phase space model. Besides medium modifications like Fermi motion and Pauli blocking we focus on the collision broadening of the involved resonances. The resonance contributions to the elementary cross section are fixed by fits to partial wave amplitudes of pion photoproduction. The cross sections for $N \, R \to N \, N$, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to $N \, N \to N \, N \, \pi$ cross sections. We show that a reasonable collision broadening is not able to explain the experimentally observed disappearance of the $D_{13}(1520)$-resonance in the photoabsorption cross section on nuclei.Comment: 26 pages Latex including 9 postscript figure