Low mass dimuon production in proton and ion induced interactions at the SPS

The low mass dimuon spectra collected in p-U collisions by the NA38 experim ent significantly exceeds the total cross section expected from previous an alysis, done by other experiments. The 'excess' events have a harder p(T) d istribution than the muon pairs from eta and omega Dalitz decays, expected to dominate the mass window 0.4-0.65 GeV/c(2). We conjecture that the exces s events may be due to q (q) over bar annihilations, negligible at low p(T) but made visible by the m(T) cut applied in the NA38 data. Taking this ass umption to parametrise the p-U spectra, we proceed with the corresponding a nalysis of the S-Cu, S-U and Pb-Pb data, collected by the NA38 and NA50 exp eriments, comparing the measured low mass dimuon spectra with the 'expected cocktail'.