Dilepton production at SPS energies

We present a dynamical study of e+e− production in proton-nucleus and nucleus-nucleus collisions at SPS energies on the basis of a covariant transport approach. For p + A reactions the dilepton yield for invariant masses m ≤ 1.2 GeV is found to be dominated by the decays of the η, ϱ, ω and Φ mesons in line with the findings of the CERES Collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is, however, dominated by π+π− annihilation due to the high pion densities achieved. For ‘free’ meson masses and form factors the experimental cross section is slightly underestimated for 0.3 GeV ≤ m ≤ 0.45 GeV. Effects due to different medium modification of the ϱ-meson are studied in comparison to the CERES data.