Analysis of the HELIOS-3 $\mu^+\mu^-$ Data within a Relativistic   Transport Approach
  We present a nonperturbative dynamical study of $\mu^+\mu^-$ production in proton-nucleus and nucleus-nucleus collisions at SPS energies on the basis of the covariant transport approach HSD. For p + W reactions at 200 GeV bombarding energy the dimuon yield for invariant masses m $\leq $ 1.6 GeV is found to be dominated by the decays of the $\eta, \rho, \omega$ and $\Phi$ mesons. For 200 GeV/A S + W collisions, however, the dimuon yield shows an additional large contribution from $\pi^+\pi^-$, $K^+K^-$ and $\pi \rho$ channels. We find that for `free' meson masses and form factors the experimental cross section is clearly underestimated for S + W in the invariant mass range 0.35 GeV $\leq$ m $\leq$ 0.65 GeV and that the HELIOS-3 data can only be reproduced within a hadronic scenario, if the $\rho/\omega$-meson mass drops with baryon density. This finding suggests a partial restoration of chiral symmetry in S + W collisions at SPS energies. 
W. Cassing, W. Ehehalt and I. Kralik