In this thesis we use a transport-model approach to study the observability of in-medium modifications of vector mesons in various setups. In the past, a number of possible medium-modification schemes have been proposed, including effects like collisional broadening, a pole-mass shift (related to a partial restoration of chiral symmetry) and more complex scenarios. However, stringent experimental evidence turned out to be extremely hard to obtain.We use the Giessen BUU model (GiBUU) to study vector-meson production in proton-nucleus, nucleus-nucleus and photon-nucleus collisions. To observe in-medium changes of these mesons, the dilepton decay mode is particularly well-suited, since the leptons do not interact strongly with the surrounding hadronic medium. As an alternative, also hadronic decay modes like omega -> pi0 gamma are used, which are subject to hadronic final-state interaction, however.After a general introduction of the model, the first part of the thesis deals with dilepton spectra from pp, pA and AA reactions in the few-GeV regime, as measured by the DLS and HADES detectors. The DLS spectra pose a long-standing puzzle and have never been fully explained to date. However, they have been confirmed experimentally by the second-generation measurements of the HADES collaboration at GSI. Our investigations aim to give new impulses to the solution of the DLS puzzle and at the same time explain the new HADES data. In particular, we achieve an improved description of the elementary pp collisions by consistently incorporating effects of the baryonic resonances, which have often been neglected or underestimated in previous studies. Such effects already proved to be essential for the in-medium self-energies of vector mesons in heavy-ion collisions at higher energies (measured by the NA60 detector). We show that at low energies especially the rho meson´s coupling to baryon resonances can influence dilepton spectra - even in elementary pp collisions. Such effects provide an important link to the high-energy results and give an additional handle on in-medium properties of vector mesons.In the second part of the thesis, we investigate the pi0-gamma decay mode of the omega meson in photoproduction experiments, as carried out by the CBELSA/TAPS collaboration. The major obstacles of this approach are the long lifetime of the omega meson as well as the hadronic final-state interaction of the decay pion. Both of these effects together result in the pi0-gamma mass spectrum being only weakly sensitive to in-medium modifications of the omega meson. However, other observables can yield more information about the omega in medium. For example, transparency-ratio measurements have been used successfully to determine the absorption strength (and thus the collisional width) by comparison to transport simulations. Our investigations fully confirm the validity of this approach. Moreover, we identify further observables that exhibit a sensitivity to in-medium modifications, such as the excitation function and the momentum spectrum of pi0-gamma pairs. Both of these are primarily sensitive to a mass shift and thus yield information complementary to the transparency ratio.
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