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 experiment significantly exceeds the total cross section expected from previous analysis, done by other experiments. The `excess' events have a harder p {T} distribution than the muon pairs from η and ω Dalitz decays, expected to dominate the mass window 0.4-0.65 GeV/ c2. We conjecture that the excess events may be due to q {q} annihilations, negligible at low p {T} but made visible by the m {T} cut applied in the NA38 data. Taking this assumption to parametrise the p-U spectra, we proceed with the corresponding analysis of the S-Cu, S-U and Pb-Pb data, collected by the NA38 and NA50 experiments, comparing the measured low mass dimuon spectra with the `expected cocktail'.