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 from the dimuon threshold up to the J/Ψ 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 ρ /ω mesons, and between the φ and J/Ψ 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+/- 4)% of the corresponding 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 low mass region follows an exponential transverse mass distribution with an average inverse slope parameter T of (190+/-5) MeV /c2, 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 /c2 the continuum dimuon mass distribution observed in p-W interactions can be quantitatively understood as a sum of three expected contributions (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 does not depend on centrality, nor very strongly on the transverse momentum.