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Angular distributions of charged particles produced in Pb-Pb collisions at the CERN SPS have been measured by the NA50 experiment. Measurements have been performed with a silicon microstrip detector at two different beam energies (40 and 158 GeV/nucleon) and over a wide impact parameter (centrality) range. The centrality of the collision has been evaluated both by neutral transverse energy released in the interaction and by spectator nucleons energy (zero degree energy). The charged particle density at midrapidity shows a linear dependence on the number of participating nucleons at both beam energies. The particle density per participant pair and its S dependence are also studied.

ψ′ production is studied in Pb–Pb collisions at 158 GeV/c per nucleon incident momentum. Absolute cross-sections are measured and production rates are investigated as a function of the centrality of the collision. The results are compared with those obtained for lighter colliding systems and also for the J/ψ meson produced under identical conditions.

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’.

The J / ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J / ψ mesons at SPS energies. Hence, the measurement of J / ψ elliptic anisotropy, quantified by the Fourier coefficient v 2 of the J / ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J / ψ suppression observed in Pb-Pb collisions. We present the measured J / ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v 2 as a function of the collision centrality and of the J / ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p T sub-samples. The extracted v 2 values are significantly larger than zero for non-central collisions and are seen to increase with p T .

We present a new determination of the ratio of cross-sections (J/$\\psi$)/DY as expected for nucleus-nucleus reactions if J/$\\psi$ would only be normally absorbed by nuclear matter. This anticipated behaviour is based on proton-nucleus data exclusively, and compared, as a function of centrality, with updated S-U results from experiment NA38 and with the most Pb-Pb results from experiment NA50.