Excess of continuum dimuon production at masses between threshold and the $J/\psi$ in S-W interactions at 200 GeV/c/nucleon

Results are presented on dimuon production for invariant masses % interval ranging from the dimuon threshold up to the $J/\Psi$ meson. Proton-tungsten and sulphur-tungsten interactions at 200 GeV $/c$/nucleon were measured over a large kinematic region, using the HELIOS/3 dimuon spectrometer at the CERN SPS. In the continuum regions between the dimuon threshold and the $\rho/\omega$ meso ns, and between the $\phi$ and $J/\Psi$ mesons, an excess is observed in S--W interactions relative to minimum bias p--W interactions. The observed excess is continuous over the explored mass range and has no apparent resonant structure. In the low mass region the dimuon yield increases by $(76\pm 4)$\% of the corresponding p--W dimuon spectrum, whereas in the higher mass region the exces s amounts to $2.4\pm 0.3$ times the p--W yield nearly independent of the charged multiplicity and rapidity. The observed excess for the low mass region follows an exponential distribution with an average inverse slope parameter $T$ of $(197\pm6)$ MeV$/c^2$, constant for all but the lowest charged multiplicity interval and consistent with the slope of the excess in the higher mass region . In the invariant mass range of 1.35 - 2.5 GeV$/c^2$ the continuum dimuon mass distribution observed in p--W interactions can be quantitatively understood as a sum of three expected contribut ions (vector meson decays, semileptonic charm decays and Drell-Yan process), but both in central and in minimum bias S--W interactions an excess is observed with respect to these sources which doe s not depend on centrality, nor very strongly on the transverse momentum.