ANALYSIS OF THE HELIOS-3 MU(-)-DATA WITHIN A RELATIVISTIC TRANSPORT APPROACH()MU()

We present a nonperturbative dynamical study of mu(+)mu(-) production in proton-nucleus and nucleus-nucleus collisions at SPS energies on th e basis of the covariant transport approach HSD. For p + W reactions a t 200 GeV bombarding energy the dimuon yield for invariant masses m le ss than or equal to 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, ho wever, the dimuon yield shows an additional large contribution from pi (+)pi(-), K+K- and pi rho channels. We find that for 'free' meson mass es and form factors the experimental cross section is clearly underest imated for S + W in the invariant mass range 0.35 GeV less than or equ al to m less than or equal to 0.65 GeV and that the HELIOS3 data can o nly be reproduced within a hadronic scenario, if the rho/omega-meson m ass drops with baryon density. This finding suggests a partial restora tion of chiral symmetry in S + W collisions at SPS energies.