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

Abstract

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 $p_{\mathrm{T}}$ distribution 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 excess events may be due to $q\bar{q}$ annihilations, negligible at low $p_{\mathrm{T}}$ but made visible by the $m_{\mathrm{T}}$ 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’.