We present a nonperturbative 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. Whereas for `free' meson masses and form factors the experimental
cross section is slightly underestimated for 0.3 GeV ≤ m ≤ 0.45 GeV,
different medium modifications of the ρ-meson appear compatible with
current CERES data.Comment: 1 compressed uuencoded postscript file with 6 figures included, 18
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