Analysis of the HELIOS-3 μ+μ- Data within a Relativistic Transport Approach

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We present a nonperturbative dynamical study of μ+μ- 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 ≤ 1.6 GeV is found to be dominated by the decays of the η, ρ, ω and Φ mesons. For 200 GeV/A S + W collisions, however, the dimuon yield shows an additional large contribution from π+π-, K+K- and πρ 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 ≤ m ≤ 0.65 GeV and that the HELIOS-3 data can only be reproduced within a hadronic scenario, if the ρ/ω-meson mass drops with baryon density. This finding suggests a partial restoration of chiral symmetry in S + W collisions at SPS energies.