M.I. Gorenstein

Excluded volume effect for the nuclear matter equation of state

We present the thermodynamically consistent procedure to introduce the excluded volume effect into the equation of state of nuclear matter. Implications are discussed in the framework of a mean-field model for hadrons with eigenvolume.

Statistical Coalescence Model with Exact Charm Conservation

The statistical coalescence model for the production of open and hidden charm is considered within the canonical ensemble formulation. The data for the J/ multiplicity in Pb+Pb collisions at 158 AGeV are used for the model prediction of the open charm yield which has not yet been measured in these reactions. The charmonium states J/ andhave been measured in nucleus-nucleus (A+A) collisions at CERN SPS over the last 15 years by the NA38 and NA50 Collaborations. This experimental program was motivated by a suggestion (1) to use the J/ as a probe of the state of matter created in the early stage of the collision. In this approach a significant suppression of J/ production relative to Drell-Yan lepton pairs is predicted when going from peripheral to central Pb+Pb interactions at 158 AGeV. This is originally attributed to the formation of a quark-gluon plasma, but could be also explained in microscopic hadron models as secondary collision effects (see (2) and references therein). The statistical approach, formulated in Ref.(3), assumes that J/ mesons are created at hadronization according to the available hadronic phase-space. In this model the J/ yield is independent of the open charm yield. The model offers a natural explanation of the proportionality of the J/ and pion yields and the magnitude of the J/ multiplicity in hadronic and nuclear collisions. Recently the statistical coalescence model (4) and the microscopical coalescence model (5) were introduced for the charmonium production. Similar to the statistical model (3), the charmonium states are assumed to be formed at the hadronization stage. However, they are produced as a coalescence of created earlier c-c quarks and therefore the multi- plicities of open and hidden charm hadrons are connected in the coalescence models. In Ref. (4) the charm quark-antiquark pairs are assumed to be created at the early stage of A+A collision and the average number of c-c pairs, N dir cc , is fixed by the model consid- eration based on the hard scattering approach. The estimated number N dir

Particle number fluctuations in a canonical ensemble

Fluctuations of charged particle number are studied in the canonical ensemble. In the infinite volume limit the fluctuations in the canonical ensemble are different from the fluctuations in the grand canonical one. Thus, the well-known equivalence of both ensembles for the average quantities does not extend for the fluctuations. In view of the possible relevance of the results for the analysis of fluctuations in nuclear collisions at high energies, a role of the limited kinematical acceptance is studied.

The analysis of particle multiplicities in Pb+Pb collisions at CERN SPS within hadron gas models

The preliminary data on hadron multiplicities measured in central Pb+Pb collisions at 158A GeV/c are analyzed. The ideal hadron gas model fails to give a reasonable explanation to the Pb+Pb data sets. We study the possible effects of pion enhancement due to different hard-core repulsion for pions and other hadrons and strangeness suppression because of incomplete chemical equilibrium. Each of these two modifications improves the results. The combined effect of these two mechanisms leads to an extremely good agreement with the data. An interpretation of the obtained results in terms of the possible quark-gluon plasma formation at the early stage of the collision is also discussed.

Exactly soluble model for nuclear liquid-gas phase transition

Thermodynamical properties of nuclear matter undergoing multifragmentation are studied within a simplified version of the statistical model. An exact analytical solution has been found for the grand canonical ensemble. Excluded volume effects are taken into account in the thermodynamically self-consistent way. In thermodynamic limit the model exhibits a first order liquid-gas phase transition with specific mixed phase properties. An extension of the model including the Fishers term is also studied. The possibility of the second order phase transition at or above the critical point is demonstrated. The fragment mass distributions in the different regions of the phase diagram are discussed.

Strangeness enhancement in heavy ion collisions: Evidence for quark gluon matter?

Abstract The centrality dependence of (multi-)strange hadron abundances is studied for Pb(158xa0Axa0GeV)Pb reactions and compared to p(158xa0GeV)Pb collisions. The microscopic transport model UrQMD is used for this analysis. The predicted Λ/π−, Ξ−/π− and Ω − /π − ratios are enhanced due to rescattering in central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. However, the enhancement is substantially smaller than observed experimentally. The enhancement depends strongly on the kinematical cuts. The maximum enhancement is predicted around midrapidity. For Λs, strangeness suppression is predicted at projectile/target rapidity. For Ω s, the predicted enhancement can be as large as one order of magnitude. Comparisons of Pb-Pb data to proton induced asymmetric (p-A) collisions are hampered due to the predicted strong asymmetry in the various rapidity distributions of the different (strange) particle species. In p-Pb collisions, strangeness is locally (in rapidity) not conserved. The present comparison to the data of the WA97 and NA49 Collaborations clearly supports the suggestion that conventional (free) hadronic scenarios are unable to describe the observed high (anti-)hyperon yields in central collisions. A reduction of the constituent quark masses to the current quark masses ms∼230 MeV, mq∼10 MeV, as motivated by chiral symmetry restoration, yields hyperon production close to the experimentally observed high values. An ad hoc overall increase of the color electric field strength (effective string tension of κ=3 GeV/fm) yields similar results. It has been suggested that these findings might be interpreted as a signal of a phase of nearly massless particles.

Local equilibrium in heavy ion collisions: Microscopic model versus statistical model analysis

The assumption of local equilibrium in relativistic heavy ion collisions at energies from 10.7 AGeV (AGS) up to 160 AGeV (SPS) is checked in the microscopic transport model. Dynamical calculations performed for a central cell in the reaction are compared to the predictions of the thermal statistical model. We find that kinetic, thermal and chemical equilibration of the expanding hadronic matter are nearly approached late in central collisions at AGS energy for

EVIDENCE FOR STATISTICAL PRODUCTION OF J/PSI MESONS IN NUCLEAR COLLISIONSAT 158-200A GEV

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Hadron-resonance gas at freeze-out: Reminder on the importance of repulsive interactions

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J/ψ suppression and enhancement in Au + Au collisions at the BNL RHIC

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