In-Medium Properties of Vector Mesons in Elementary Nuclear Reactions

While the vacuum properties of most hadrons are known to reasonable accuracy today, it is a much-debated question how these properties change in nuclear matter. In particular for the light vector mesons there have been various theoretical predictions regarding their in-medium properties. One of the expected in-medium effects is the so-called ’collisinal broadening’ of the meson spectral function, due to inleastic collisions with the hadronic medium. A second class of predictions claim that the vector meson masses will be shifted in the medium [1]. These changes of the peak mass are connected to the partial restoration of chiral symmetry in the medium and have been studied via QCD sum rules [2]. This effect has been claimed to be seen in experiments, but is still being discussed controversially. For studying in-medium effects, the more promiment hadronic decay modes of the vector mesons suffer from the drawback that they are affected by strong final-state interaction with the hadronic medium – in contrast to the rare dilepton decay modes, which only feel the electromagnetic force. Therefore they are ideally suited to carry the inmedium information outside to the detector, undisturbed by the hadronic medium. Although dilepton production has first been studied in heavy-ion collisions, we concentrate on elementary nuclear reactions in our work (γA, pA and πA). These have a number of advantages, in particular they involve a more sharply defined density and are simply much cleaner. This makes it easier to interpret the resulting spectra and to disentangle the various contributing channels. Experimentally, dilepton spectra from elementary nuclear reactions are being studied e.g. with the CLAS detector at JLAB, where Bremsstrahlung photons in the energy range of about 1-3 GeV are shot on nuclei [3], or by the E325 experiment at the Japanese KEK facility, where 12 GeV protons are used as projectile [4]. Also the HADES detector at GSI has an ambitious program for measuring dilepton spectra from AA, pA and pp reactions [5]. On the side of the hadronic decays, most notably ω → π γ is being investigated by the CB/TAPS group in photon-induced reactions at the ELSA accelerator [6]. For the numerical simulation of these processes we use the GiBUU transport model, which provides a unified framework for various types of elementary reactions on nuclei as well as heavy-ion collisions [7, 8, 9]. This model takes care of the correct transport-theoretical description of the hadronic degrees of freedom in nuclear reactions and all the subtleties this may involve, including the propagation, collisions and decays of particles in a nuclear envi-