A Monte Carlo model to simulate nuclear collisions in the energy range going from SPS to LHC, is presented. The model includes in its initial stage both soft and semihard components, which lead to the formation of color strings. Collectivity is taken into account considering the possibility of strings in color representations higher than triplet or antitriplet, by means of string fusion. String breaking leads to the production of secondaries. At this point, the model can be used as initial condition for further evolution by a transport model. In order to tune the parameters and see the results in nucleus-nucleus collisions, a naif model for rescattering of secondaries is introduced. Results of the model are compared with experimental data, and predictions for RHIC and LHC are shown.

We show that, even in purely soft processes, the hadronic multiplicity in nucleusnucleus interactions contains a term that scales with the number of binary collisions.In the absence of shadowing corrections, this term dominates at mid rapidities and high energies. Shadowing corrections are calculated as a function of impact parameter and the centrality dependence of mid-rapidity multiplicities is determined.The multiplicity per participant increases with centrality with a rate that increases between SPS and RHIC energies, in agreement with experiment.

We study the ratio of J/ψ over minimum bias in P b P b collisions at SPS energy. The NA50 data exhibit a sharp turn-over at E T ∼ 100 GeV (close to the knee of the E T distribution) followed by a steady, steep decrease at larger E T . We show that this behaviour can be explained by the combined effects of a small decrease of the hadronic E T in the J/ψ event sample (due to the E T taken by the J/ψ trigger), together with the sharp decrease of the E T distributions in this E T region (tail). This phenomenon does not affect the (true) ratio J/ψ over DY (obtained by the NA50 standard analysis), but does affect the one obtained by the so-called minimum bias analysis. A good agreement is obtained with the data coming from both analysis -as well as with the ratios of J/ψ and DY over minimum bias -in the whole E T region.