Analysis of the HELIOS-3 ${\mu }^{+}{\mu }^{-}$ Data within a Relativistic Transport Approach

Abstract

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 $\le$ 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 $\le$ m $\le$ 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.

Comments: 11 pages, 5 postscript figures