Starting from a realistic one-boson-exchange-model fitted to the amplitudes of elastic nucleon-nucleon scattering and the process N N → N ∆ we perform a fully relativistic and gauge invariant calculation for the dilepton production in nucleon-nucleon collisions, including the important effect of propagating the ∆-resonance. We compare the results of our calculations with the latest experimental data on dilepton production. We also show how to implement various electromagnetic formfactors for the hadrons in our calculations without loosing gauge-invariance and discuss their influence on dilepton spectra. * Supported by BMFT and GSI Darmstadt † Part of dissertation of M. Schäfer 0

A microscopic model of the electromagnetic form factor of the nucleon is developed in a hadronic framework, including pions, nucleons, and the E resonance explicitly. The spacelike on-shell form factors are reproduced and predictions for the half off-shell dependence are made. The impact of this off-shell dependence in the timelike sector (q ( 1 GeV, thus including the region of vector meson dominance) is of main interest in this investigation.PACS number(s): 13.40. Gp, 14.20.Dh

submitted to Physics Letter BWe explore the possibility to verify vector meson dominance for the nucleon by measuring the half-off-shell form factor in the vector-meson mass region.Cross sections for the process γp → p + e + e − be presented. * Work supported by GSI Darmstadt and BMFT † Part of dissertation of M. Schäfer 0