Michael Weber (University of Houston, Texas, USA)
Recently the HADES collaboration has extended its physics program from heavy ion collisions to proton induced reactions in order to investigate cold nuclear matter effects. Therefore we did two experiments, p+p and p+Nb, focusing on the modification of the spectral function in the rho/omega meson invariant mass region. Compared to previous measurements the high acceptance of the HADES detector allows for a detailed analysis of e+e- pairs with low momenta relative to the medium, where predicted modifications of the spectral functions of vector mesons are believed to be most prominent. Comparing slow and fast e+e- pairs from the same data sample we find a strong difference in the shape of the spectral distribution in the whole vector meson region pointing towards a significant reduction of the omega yield around its pole mass for slow pairs and an enhanced contribution from rho and Delta decays.