Particle-hole excitations in nucleus-nucleus collisions

Nucleus-nucleus collisions lead to nuclear excitations which can be expressed in terms of particles and holes. An empirical formula is presented to calculate this initially degree of freedom. The formula is tested against data and found to have a large range of validity with respect to projectile masses and bombarding energies.     

Higgs-boson production in nucleus-nucleus collisions

Cross-section calculations are presented for the production of intermediate-mass Higgs bosons produced in ultrarelativistic nucleus-nucleus collisions via two-photon fusion. The calculations are performed in position space using Baur's method for folding together the Weizsacker-Williams virtual-photon spectra of the two colliding nuclei. It is found that two-photon fusion in nucleus-nucleus collisions is a plausible way of finding intermediate-mass Higgs bosons at the Superconducting Super Collider or the CERN Large Hadron Collider.     

Onset of deconfinement in nucleus-nucleus collisions

The energy dependence of hadron production in relativistic nucleus-nucleus collisions reveals anomalies-the kink, horn, and step. They were predicted as signals of the deconfinement phase transition and observed by the NA49 Collaboration in central PbPb collisions at the CERN SPS. This indicates the onset of the deconfinement in nucleus-nucleus collisions at about 30 A GeV.     

Dynamical aspects of nucleus-nucleus collisions

This is an extension of the macroscopic theory of nucleus-nucleus reactions described by Swiatecki. The fusion or reseparation of two colliding nuclei is treated according to a schematic model based on the “chaotic regime dynamics” (liquid-drop potential energy plus one-body dissipation). Attention is focused on three hurdles or “milestone configurations” that a colliding system may be faced with: the touching configuration, the conditional saddle-point configuration at frozen mass asymmetry, and the unconditional saddle-point configuration. Semi-empirical formulae are derived for the “extra push” (the extra energy needed in some situations to carry the system from the first to the second hurdle) and for the “extra-extra push” (the energy needed to carry the system from the first to the third hurdle). The theoretical formulae are confronted with measurements of fusion and evaporation-residue cross sections. A discussion of the implications for super-heavy-element reactions is given, using the production of element 107 in the bombardment of ²⁰⁹Bi with ⁵⁴Cr as a calibrating reaction.     

Event selection criteria in nucleus-nucleus collisions

Several event selection criteria in ultrarelativistic nucleus-nucleus collisions are analysed in the framework of the dual parton model. First, central and semicentral collisions are compared. We conclude that semicentral processes are more appropriate to observe high energy density events, where the formation of a quark-gluon plasma could take place. Secondly, the dependence of the achieved energy density on the degree of centrality of the collision is studied. We analyse the implications of our results for future experiments at CERN.     

Baryon distributions in ultrarelativistic nucleus-nucleus collisions

We have measured distributions in transverse momentum and rapidity of protons from interactions of 14.6 GeV/nucleon²⁸Si projectiles with targets of Al and Pb. The transverse momentum spectra exhibit a thermal shape with a rapidity dependent temperature parameter. For very central or violent collisions the proton rapidity distributions exhibit the large rapidity shifts and (for Si+Al) a peak at midrapidity as required for full stopping.     

Multiplicity fluctuations in relativistic nucleus-nucleus collisions

We discuss the event-by-event multiplicity fluctuations in relativistic nucleus-nucleus collisions. Recent results of the transport and statistical approaches are presented and compared with existing data. The text was submitted by the author in English.     

Multifractal critical thermodynamics in nucleus-nucleus collisions

Critical points approach in the frames of multifractal thermodynamics is suggested to interpret the experimental data on nuclear multifragmentation which come from interactions in nuclear emulsion (in which ¹⁹⁷ ₇₉Au₁₁₈ nuclei of energy ∼1 GeV/nucleon break up into fragments) and from the charge distributions of projectile fragments in sulphur (³²S) fragmentation at 200 GeV/nucleon. It is also shown that multifragmentation after macro-solids collisions exhibits properties analogous to those observed in the nuclear multifragmentation experiments. Received: 19 October 1999     

Pion multiplicity in central nucleus-nucleus collisions

Recently, the discrepancy between standard intranuclear cascade model predictions and experimental data for the pion multiplicity in central nucleus-nucleus collisions has been interpreted as a manifestation of compression effects. We explore here alternative effects, involving pion reabsorption by two nucleons, off-mass shell effects and renormalization of the pion sources inside a dense nuclear medium.     

Prompt emission of nucleons in heavy-ion collisions

The mechanism of the production of quasifree promptly emitted particles in heavy ion collisions is studied within a sharp surface model. The particle production cross section is found to be rather low, ～1% of the total reaction cross section at energies slightly above the Coulomb barrier. Arguments on how to observe them in experiments are presented. Comparison between theoretical results and experimental data does not rule out the possibility of this reaction process.     

Rescattering production of dileptons in nucleus-nucleus collisions

We present a space-time formulation for the rescattering of hard partons produced in primary QCD subcollisions within a nucleus-nucleus collision. As an application we calculate the rescattering rate for dilepton production in 100 GeV/A+100 GeV/A uranium-uranium collisions.     

Theory of fragmentation dynamics in nucleus-nucleus collisions

The fragmentation of a nuclear system is described by a dynamical coordinate, the fragmentation coordinate η=A₁?A₂/A₁+A₂ The total Hamiltonian describing a binary heavy ion encounter includes a part H(R, α,η) which describes the fragmentation and its coupling to other degrees of freedom. It is a collective Hamiltonian which is calculated from the microscopic Asymmetric Two-Center Shell Model (ATCSM). The properties of this Hamiltonian are extensively discussed and methods for its solution are described. Formulae for cross sections of specific nuclear fragmentations including fusion are derived.     

Analytical treatment of high-energy nucleus-nucleus collisions

We review an analytical study of high-energy nucleus-nucleus collisions based on Boltzmann's kinetic theory of gases. In the first part of the paper, we do not take effects of the nuclear mean field into account. This turns out to be appropriate for particle inclusive production, as the good agreement of the theory with a large variety of inclusive experimental data demonstrates. As soon as physical observables extracted from measurements on an event per event basis are considered, the nuclear potential plays a decisive rôle. To account for this fact, we extend the theory by including the nuclear mean field in the Boltzmann equation. The results obtained are in qualitative agreement with experimental exclusive observables such as the sideward kinetic energy flow angle and the average transverse momentum, despite the absence of strong compression of nuclear matter in the theory presented.     

Dissipative diabatic dynamics of nucleus-nucleus collisions

First numerical studies of the time-dependence of central nucleus-nucleus collisions within the dissipative diabatic approach are presented. Effects from the non-markovian behaviour of dissipation, i.e., from the partially elastic response are illustrated and discussed for⁴⁰Ca +⁴⁰Ca and⁹⁰Zr +⁹⁰Zr at various incident energies. In particular, the probability for the emission of precompound giant quadrupole gammas in the fusion process of intermediate-mass nuclei is estimated.     

Dynamical aspects of intermediate-energy nucleus-nucleus collisions

Central collisions of heavy nuclei are investigated within the limit of selfconsistent independent single-particle motion in terms of the Wigner transform of the one-body density matrix in space and time. The energy regime considered extends from roughly 15 MeV/u to 150 MeV/u. Special attention is devoted to the relation between energy and momentum of the nucleons in the collision zone of the two nuclei. An initial average energy-momentum distributionf(k;E) is determined within the fully quantal approach which proves to be vital for our understanding of energetic particle production in the very early stage of the heavy-ion reaction. A comparison with related classical quantities shows the peculiar features of quantum dynamics for finite size fermion systems far from equilibrium.