Non-equilibrium processes in heavy-ion collisions at relativistic energies

We use transport theory to describe the inclusive cross sections for protons and pions produced in collisions between two identical heavy ions at an energy of 800 MeV per particle. In addition to the nucleonic we take the Δ-degree of freedom into account. Thus we consider a two-component system whose distributions in transverse momentum and rapidity we describe by two coupled Fokker-Planck equations. These transport equations contain the one-nucleon knock-out process as initial condition. In the limit of large interaction times they lead to thermal equilibrium (fireball) distributions. For light nuclei the interaction time is not large enough for equilibrium to be reached. A recent experiment for two colliding carbon nuclei at 800 MeV per nucleon shows evidence of nonequilibrium effects. We compare our calculations with experimental data for ¹²C on ¹²C and Ne on NaF at 800 MeV/N.     

The Hadron to quark-gluon transition in relativistic heavy-ion collisions

In this paper we construct a scenario for the QCD transition from the hadron phase to the quark/gluon phase using physical models for these phases. The hadron phase is modeled by a spectrum of hadrons with masses which drop (with a common scaling factor) towards zero at chiral symmetry restoration. The number of hadronic effective degrees of freedom is limited by the number of microscopic degrees of freedom in the quark/gluon phase. This limitation can be imposed either by fiat or through the introduction of a temperature-dependent excluded volume. Given that the number of degrees of freedom in hadrons and in quarks and gluons are roughly equal, the QCD phase transition is inhibited by the bag constant. The only phase transition seen in lattice-gauge calculations, once low-mass quarks are included, is the restoration of chiral symmetry which occurs at the relatively low temperature of ˜ 150 MeV. At present, lattice gauge calculations do not have the resolution to determine the properties of the higher hadronic states accurately. They do, however, demonstrate that chiral restoration takes place in the (ρ. a₁), ( +)), ( −)) and (π, σ) systems by yielding “screening masses” for chiral partners which are distinct for T < T _xSR and identical for T>T _xSR. Further, within numerical accuracy, these “screening masses” are consistent with pure thermal energies and show no evidence of remaining bare masses once chiral symmetry is restored. These, and other lattice-gauge results, will be discussed in the light of our scenario. We shall also consider the consequences of our picture for relativistic heavy-ion experiments.     

Relativistic effects in heavy-ion collisions at SIS energies

The covariant and non-covariant Quantum Molecular Dynamics models are applied to investigate possible relativistic effects in heavy ion collisions at SIS energies. These relativistic effects which arise due to the full covariant treatment of the dynamics are studied at bombarding energiesE _lab=50, 250, 500, 750, 1000, 1250, 1500, 1750 and 2000 MeV/nucl. A wide range of the impact parameter fromb=0 fm tob=10 fm is also considsered. In the present study, five systems¹²C-¹²C,¹⁶O-¹⁶O,²⁰Ne-²⁰Ne,²⁸Si-²⁸Si and⁴⁰Ca-⁴⁰Ca are investigated. The full covariant treatment at low energies shows quite good agreement with the corresponding non-covariant whereas at higher energies it shows less stopping and hence less thermal equilibrium as compared to the non-covariant approach. The collisions dynamics is less affected. The density using RQMD rises and drops faster than with QMD. The relativistic effects show some influence on the resonance matter production. Overall, the relativistic effects at SIS energies (≦2000 MeV/nucl.) are less significant.     

Production mechanism of quark-gluon plasma in heavy-ion collisions

A general scheme is proposed here to describe the production of semihard and soft quarks and gluons that form the bulk of the plasma in ultrarelativistic heavy-ion collisions. We show how to obtain the production rates in the extended phase space, including the color part, as a function of time in a consistent manner and without having to make ad hoc assumptions. All the required features-the back reaction on QCD vacuum, the non-Markovian nature of the production, and the quasi particle nature of the partons-are naturally incorporated. We illustrate the results with a realistic albeit toy model and also show how physically tenable source terms may be obtained.     

A four-channel unitary model for the description of heavy-ion collisions at relativistic energies

We expand a previously formulated model for nuclear abrasion in ion-ion collisions where now nucleons in both projectile and target can be excited. Describing the state of excitation by an effective channel we rigorously impose unitarity and compare the results with those of a heuristic treatment of unitarity by Hüfner, Schäfer and Schürmann. We find corrections to the latter theory which grow up to a factor of two with the number of abraded nucleons.     

Clusterisation and isospin effects in heavy-ion collisions at intermediate energies

We study the multifragmentation phenomenon in heavy-ion collisions by varying the spatial constraint criterion in minimum spanning tree (MST) clusterisation procedure. Within the framework of isospin-dependent quantum molecular dynamics (IQMD) model, the role of isospin-dependent spatial constraint, i.e. iso-MST version, is investigated on different fragment observables in various isobaric pair of reaction systems varying in the entrance channel isospin (N / Z) content. The fragment observables such as persistence, gain, average yield of free nucleons, light and intermediate mass fragments are slightly sensitive to the isospin-dependent spatial constraint criterion particularly in heavier reaction systems. For a given isobaric pair of reaction systems, the fragment production, however, remains indifferent to isospin content of the colliding nuclei.     

Nucleus-nucleus collisions at relativistic energies

For relativistic beam nuclei up to ⁴He and medium heavy target nuclei, absorption cross sections and partial production cross sections are computed. Eclipse corrections are found to be essential. For C, N, and O beam nuclei, Coulomb dissociation is computed.     

Pion-production in heavy-ion collisions at SIS energies

We investigate the production of pions in heavyion collisions in the energy range of 1–2 GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides theN(938) and theΔ(1232) we also take into account nucleon resonances up to masses of 1.95 GeV/c² as well asπ-,η- andρ-mesons. We study in detail the influence of the higher baryonic resonances and the 2π-production channels (NN→NNππ) on the pion spectra in comparison toπ ^? data fromAr+KCl collisions at 1.8 GeV/A andπ ⁰-data forAu+Au at 1.0GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data forAr+KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reaction dynamics is well described by our novel transport approach.     

Charged particle density distributions in Au + Au collisions at relativistic heavy-ion collider energies

Charged particle pseudorapidity distributions have been measured in Au+ Au collisions using the BRAHMS detector at RHIC. The results are presented as a function of the collision centrality and the center of mass energy. They are compared to the predictions of different parton scattering models and the important role of hard scattering processes at RHIC energies is discussed. Keywords. Relativistic heavy-ion collisions; charged hadron production; pseudorapidity distributions; centrality dependence; hard scattering processes.     

Photon production in heavy-ion collisions at SPS energies

Single photon spectra in heavy-ion collisions at SPS energies are studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. We separate the total photon spectrum into “background” arising from the radiative decays of π⁰ and η mesons, and the “thermal” one from other sources. For the latter we include contributions from radiative decays of ρ, ω, η′, and a₁, radiative decays of baryon resonances, as well as two-body processes such as ππ → ργ and πρ → πγ. It is found that more than 95% of all photons come from the decays of π⁰ and η mesons, while the thermal photons account for less than 5% of the total photon yield. The thermal photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper bound set by the experimental measurement of the WA80 Collaboration.     

Antiproton production in heavy-ion collisions at subthreshold energies

Within the framework of the Lanzhou quantum molecular dynamics model,the deep subthreshold antiproton production in heavy-ion collisions has been investigated thoroughly.The elastic scattering,annihilation and charge exchange reactions associated with antiproton channels are implemented in the model.The attractive antiproton potential extracted from the G-parity transformation of nucleon selfenergies reduces the threshold energies in meson-baryon and baryon-baryon collisions,and consequently enhances the antiproton yields to some extent.The calculated invariant spectra are consistent with the available experimental data.The primordial antiproton yields increase with the mass number of the colliding system.However,annihilation reactions reduce the antiproton production which becomes independent of the colliding partners.Anti-flow phenomena of antiprotons correlated with the mean field potential and annihilation mechanism is found by comparing them with the proton flows.Possible experiments at the high-intensity heavy-ion accelerator facility(HIAF) in China are discussed.     

Quarkochemistry in relativistic heavy-ion collisions

The time necessary to achieve the equilibrium ratio of strange to non-strange quarks in heavy-ion reactions is estimated in the framework of perturbative QCD. It is found, in the present approximation, to be much larger than the total collision time of even a central U + U collision at E_lab=2.1 GeV/nucleon bombarding energy.     

Intermittency in relativistic heavy-ion collisions

Corrected scaled-factorial-moments are calculated for the multiplicity distributions of shower particles in different phase spaces. An intermittency power-law is observed in the central collisions of³²S and¹⁶O at 200A GeV,¹⁶O at 60A GeV and²⁸Si at 14.5A GeV in nuclear emulsion.     

Anomalons in relativistic heavy-ion collisions

By reviewing experimental results from many anomalon searches, one can reasonably conclude that anomalons, if they exist, are preferentially produced in peripheral collisions, signified by low values of N_h and δZ. For Z ≥ 3, the experimental picture about the existence of anomalons is not very clear at this moment. In the Z = 2 case, however, there are reasonably convincing indications that anomalons do seem to be present in fragments produced by light projectile nuclei such as ¹²C.