Probing theρspectral function in hot and dense nuclear matter by dileptons

We present a dynamical study of $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $\rho$-meson spectral function that includes the pion modifications in the nuclear medium as well as the polarization of the $\rho$ meson due to resonant $\rho -N$ scattering. We find that the experimental data from the CERES and HELIOS-3 Collaborations can be described equally well as within the dropping $\rho$-mass scenario. Whereas corresponding dilepton $q_T$ spectra are found to be very similar, the inclusive dilepton yield in the invariant mass range $0.85<~M<~1.0$ GeV should allow us to disentangle the two scenarios experimentally.