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 ranging fro m the dimuon threshold up to the J/Psi meson. Proton-tungsten and sulphur-t ungsten interactions at 200 GeV/c/nucleon were measured over a large kinema tic region, using the HELIOS/3 dimuon spectrometer at the CERN SPS. In the continuum regions between the dimuon threshold and the rho/omega mesons, an d between the phi and J/Psi mesons, an excess is observed in S-W interactio ns relative to minimum bias p-W interactions. The observed excess is contin uous over the explored mass range and has no apparent resonant structure. I n the low mass region the dimuon yield increases by (76 +/- 4)% of the corr esponding p-W dimuon spectrum, whereas in the higher mass region the excess amounts to 2.4 +/- 0.3 times the p-W yield. The observed excess for the lo w mass region follows an exponential transverse mass distribution with an a verage inverse slope parameter T of (190 +/- 5) 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 1.3 5-2.5 GeV/c(2) the continuum dimuon mass distribution observed in p-W inter actions can be quantitatively understood as a sum of three expected contrib utions (vector meson decays, semileptonic charm decays and Drell-Yan proces s), but both in central and in minimum bias S-W interactions an excess is o bserved with respect to these sources which does not depend on centrality, nor very strongly on the transverse momentum.