Analysis of the first RHIC results in the string fusion model (original) (raw)

First results from RHIC on charged multiplicities, evolution of multiplicities with centrality, particle ratios and transverse momentum distributions in central and minimum bias collisions, are analyzed in a string model which includes hard collisions, collectivity in the initial state considered as string fusion, and rescattering of the produced secondaries. Multiplicities and their evolution with centrality are successfully reproduced. Transverse momentum distributions in the model show a larger p T -tail than experimental data for central collisions, and this disagreement tends to disappear with decreasing centrality. Discrepancies with particle ratios are examined comparing with previous features of the model at SPS. With the first collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL in June 2000, the study of nuclear collisions has entered the truly ultrarelativistic domain. While there exist predictions from many models , now experiments have presented results on several aspects of data, most of them corresponding to AuAu collisions at 130 GeV per nucleon in the center of mass. So it comes the time to examine the ability of models for ultrarelativistic heavy ion collisions, fitted to describe nuclear data at the much lower energies of the Super Proton Synchrotron (SPS) at CERN and nucleon data in the range of energies going from SPS to TeVatron at FNAL, to describe the new situation, and whether the evidences of Quark Gluon Plasma (QGP) already obtained at SPS are verified or not . The aim of this letter is to compare the results of the String Fusion Model (SFM) with some of the first RHIC data. Other comparisons can be found in 1 . After a very brief model description, charged multiplicities at midpseudorapidity in central collisions, evolution of charged multiplicities at midpseudorapidity with centrality, transverse momentum distributions of charged particles at different centralities and ratios of different particles will be compared with available data coming from the experiments. Finally some conclusions will be summarized. An exhaustive description of the model can be found in . Its main features are the following: Elementary inelastic collisions (binary nucleon-nucleon collisions) are considered as collisions between partons from nucleons of the projectile and the target, distributed in the transverse plane of the global collision. Some of these elementary collisions are taken as hard ones, and proceed as gluon-gluon -→ gluon-gluon through PYTHIA with GRV 94 LO parton density functions (pdf's) and EKS98 modification of pdf's inside nuclei , with subsequent radiation and fragmentation performed by ARIADNE [20] and JETSET . Those collisions not being considered hard produce soft strings in pairs. These strings are allowed to fuse if their parent partons are close enough in impact parameter ; as the number of strings increases with increasing energy, atomic number and centrality, this mechanism accordingly grows in importance. Fragmentation of soft strings is performed using the tunneling mechanism for mass and transverse momentum distributions, while longitudinal momenta are simulated by an invariant area law. The main consequences of string fusion are a reduction of multiplicities in the central rapidity region and an increase in heavy particle production. The produced particles are allowed to rescatter (between themselves and with spectators nucleons) using a very naive model with no proper space-time evolution, whose consequences are a small multiplicity reduction, an increase in strange and multistrange baryons and nucleon annihilation. Some comments are in order at this point: First, partons which generate both soft and hard strings can be valence quarks and diquarks, and sea quarks and antiquarks, so the number of soft strings is not simply proportional to the number of wounded nucleons but has some proportionality, increasing with increasing energy, centrality and nuclear size, on the number of binary nucleon-nucleon collisions 2 . Besides, only fusion of two strings in considered in the actual version of the model, and hard strings are not fused. Finally, the rescattering model is simplistic and has been included just to estimate the effects that such kind of physics could have and to tune the parameters of the model as an initial condition for a more sophisticated evolution; thus, results depending strongly on it should be taken with caution. All these aspects will be commented more extensively when the comparison with experimental data is performed. In Fig. results of the model (unless otherwise stated, results of the model correspond to its default version with the mentioned pdf's and string fusion and rescattering, see [14]) for the pseudorapidity distribution of charged particles in central collisions at SPS and RHIC are compared with experimental data. For central AuAu collisions at 130 GeV per nucleon in the center of mass, the model successfully reproduce the data, while at 56 GeV it overestimates the PHOBOS results [2]. Nevertheless, the situation at these energies is not clear: WA98 results [22] at SPS lie above the PHOBOS data at 56 GeV, and far above NA49 data [21] (as extracted in [2]) at SPS; NA49 results on multiplicities in central PbPb collisions at SPS are in agreement with those from WA97 [23]. So it is difficult to conclude anything definitive on the evolution of multiplicities with increasing energy in the model. Recently it has been proposed [24] that the evolution of multiplicities with centrality can be used as a tool to discriminate among several models for multiparticle production 2 Usually the soft contribution is taken as proportional to the number of wounded nucleons, while the contribution proportional to the number of binary nucleon-nucleon collisions is considered hard. Let us stress that this is a misleading (model dependent) statement: some proportionality with the number of binary nucleon-nucleon collisions is demanded by a basic requirement of the theory as unitarity, and has nothing to do with the soft or hard origin of these binary nucleon-nucleon collisions.