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.

The study of backward-forward and forward-forward correlations in collisions of two nuclei at high energies allow us to distinguish between the fusion of strings produced in the collision into new strings of higher color and the possibility of the fusion of produced hadrons into clusters. The results for AB collisions at the CERN Super Proton Synchrotron (SPS) and the Brookhaven Relativistic Heavy Ion Collider (RHIC) are discussed. PACS numbers: 25.75.+r, 12.38.Mh, 13.85.Ni, 24.85.+p In the last few years many efforts have been made in the search for the quark-gluon plasma [1]. One of the main points in this search is to know how the quarkgluon plasma (QGP) can be reached in the framework of the usual models of hadronic interactions [2 -4]. In these models, strings or Pomerons are exchanged between the projectile and target. The number of strings grows with the energy and with the number of nucleons of the participant nuclei. Recently, using this kind of model, two different ways to reach the QGP have been proposed.In one of them [5,6], elaborated in the framework of the vENUS model [4], the strings produced in a nucleusnucleus collision break, forming resonances and particles, which, whenever they come closer than a given radius, fuse, forming a cluster. Afterwards, this cluster decays into resonances and particles isotropically.The critical radius is fixed by comparison to the data at gszz = 19. 4 GeV. The distribution in the number of clusters as a function of the volume size is peaked at small values of the volume, as it was expected, but the probability of obtaining clusters of large volume is not negligible.In the other proposal [7,8], the strings produced in a nucleus-nucleus collision fuse if they overlap in impact parameter space, forming a new string which has a higher color charge at its ends, corresponding to the summation of the color charges located at the ends of the original strings. Then the new strings break into hadrons according to their higher color. As a result, heavy flavor is produced more efficiently and there is a reduction of the total multiplicity [9]. New strings, like the ones proposed in Refs. [7,8], have also been found[10]by studying the diffractive process y(02) + quark X + quark. In a determined kinematic range, the leading diagram is a typical triple Pomeron diagram, but the Pomeron corresponding to the discontinuity through the X system is formed by two coupled ladders of gluons, instead of one ladder as with the other two Pomerons. In the first approach, the fusion of resonances does not modify the distribution in the number of strings. There-fore there is no variation in the long range correlations. A measurement of such correlations is the backward-forward dispersion DsF = (nsnF) -(ns)(nF), (I) where ns (nF) is the number of particles in a backward (forward) rapidity range. Indeed, in any model based on a superposition of independent exchanges (Pomerons or strings), in such a way that distinct "elementary" exchanges are statistically independent, is easy to show that the...

Strangeness production at the CERN Super Proton Synchroton energies is studied within the quark-gluon string model. The observed shape of rapidity and transverse mass distributions are reproduced fairly well for both peripheral and central heavy ion collisions. However, for central collisions the model underpredicts strange particle abundances by a factor of about 2:2:4 for K&, A, and A, respectively. This discrepancy can be considered as a possible manifestation of collective string-string interactions similar to the formation of a color rope. Model predictions for coming experiments with the Pb beam at CERN are given. PACS number(s): 25.75.+r, 24.85. +p, 25.40.h, 25.40.Ve

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