Excess of continuum dimuon production at masses between threshold and the

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.