Diffusion model for relativistic heavy-ion collisions

The decay of 2.4 h^117g Cd and 3.4 h^117m Cd has been investigated with the use of high-resolution Si(Li) and Ge(Li) detectors, also in an anti-Compton arrangement. Spectra from the two isomers have been resolved by comparing young reactor-produced¹¹⁷Cd with aged cyclotron-produced¹¹⁷Cd. Ge(Li)-Ge(Li) coincidences have been measured with a multi-parameter analyzer. A hundred observedγ-rays are placed in a level scheme of¹¹⁷In. Possible theoretical interpretations are discussed. The I.T. Branching of¹¹⁷In is (47.1±1.5)%. Only about 1% of 11/2^{? 117m} Cd decays to 1.93 h 1/2^{? 117m} In and 10.9% of 1/2^{+ 117g} Cd decays to the 9/2^{+ 117g} In ground state bypassing the isomeric In state.     

HYDJET++ model of relativistic heavy-ion collisions

The HYDJET++ event generator intended for simulating multiparticle hadron production in relativistic heavy-ion collisions over a broad energy range ($ sqrt s sim 10 - 10 000 GeV $ sqrt s sim 10 - 10 000 GeV per nucleon pair) is described in the present article. Within HYDJET++, the final state of a nuclear reaction is a superposition of two independent components: a soft, hydrodynamic, part (processes involving the production of low-transverse-momentum hadrons) and a hard, multiparton, state (processes involving the production of high-transverse-momentum hadrons via the fragmentation of quark-gluon jets). Some results of a simulation of nucleus-nucleus interactions on the basis of the HYDJET++ event generator at the energies of the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) are discussed.     

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.     

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.     

Strangeness production in relativistic heavy-ion collisions

Kaon production in relativistic heavy-ion collisions is studied. Particular attention is paid to situations in which high densities are obtained, such as in the Brookhaven AGS experiments with 14.6 GeV/nucleon Si on Au. Because of the explicit chiral-symmetry breaking terms in chiral Langrangians, kaons acquire an effective mass m _K ^* which goes to zero at the critical baryon density. Well before such densities, m _K ^* is sufficiently reduced to greatly facilitate kaon production through processes like K¯K. Previous expressions for the decreasing kaon mass were arrived at by linear chiral perturbation theory. Whereas we cannot systematically proceed to higher order, we use physical models to suggest how relevant quantities will behave in higher order. We present arguments that m _K ^* effectively goes to zero in the present AGS experiments.Supported in part by the U.S. Department of Energy Grant No. DE-FG02-88ER40388     

Collective flow in relativistic heavy-ion collisions

A brief introduction is given to the field of collective flow, currently being investigated experimentally at the Relativistic Heavy-Ion Collider, Brookhaven National Laboratory. It is followed by an outline of the work that I have been doing in this field, in collaboration with Nicolas Borghini and Jean-Yves Ollitrault.     

Observing B-violation in relativistic heavy-ion collisions

Under certain situations, partons formed in heavy-ion collision experiments may expand out forming a shell-like structure. The partons in the outer shell subsequently hadronize, leaving a bubble of pure deconfined vacuum for a first-order quark-hadron phase transition. The bubble collapses and may eventually decay into particles which may thermalize to temperatures exceeding the electroweak transition temperature (100 GeV) at LHC. This will lead to the possibility of unsuppressed electroweak baryon number violating processes.     

Pion collectivity in relativistic heavy-ion collisions

The inelastic nucleon-nucleon cross section may be enhanced in the nuclear medium by pion collectivity. Empirical hamiltonians are on the borderline of strongly collective behavior, and the predicted enhancement factors range from 1–5 for various hamiltonians from the literature. An enhancement would have a substantial effect on the sideward flow in relativistic heavy-ion collisions. If this effect is present, the equation of state could be softer than in previous analyses.     

Isospin equilibration in relativistic heavy-ion collisions

We study the mixing and the kinetic equilibration of projectile and target nucleons in relativistic heavy-ion collisions in the energy regime between 150 AMeV and 2 AGeV in a coupled-channel BUU (CBUU) approach. We find that equilibrium in the projectile-target degrees of freedom is in general not reached even for large systems at low energy where elastic nucleon-nucleon collisions dominate. Inelastic nucleon excitations are more favorable for equilibration and their relative abundance increases both with energy and mass. Experimentally, the projectile/target admixture can be determined by measuring the degree of isospin equilibration in isospin asymmetric nuclear collisions. For one of the most promising systems currently under investigation, ⁹⁶ ₄₄Ru +⁹⁶ ₄₀Zr, we investigate the influence of the equation of state and the inelastic in-medium cross section. Received: 23 September 1998 / Revised version: 3 December 1998     

Hubble-like flows in relativistic heavy-ion collisions

We study the dynamical appearance of scaling solutions in relativistic hydrodynamics. The phase transition effects are included through the temperature dependent sound velocity. If a pre-equilibrium transverse flow is included in the initial conditions, then it may reach the form of the asymptotic Hubble flow, r/t, in short evolution times, 7–15 fm. The numerical solutions are found to support the freeze-out models (Blast-Wave, Buda-Lund, Cracow).     

Justification of Landau Hydrodynamic-Tube model in central relativistic heavy-ion collisions

The statistical event-by-event analysis of inelastic interactions of ¹⁶O and ³²S nuclei in emulsion at 60 A?GeV/c and 200 A?GeV/c reveals the existence of groups of high multiplicity events belonging to very central nuclear interactions with Gaussian pseudorapidity distributions for produced particles as suggested by the original hydrodynamic-tube model. Characteristics of these events are presented. The experimental observations are interpreted as a result of quark-gluon plasma formation in the course of central nuclear interactions.     

Vector meson radiation in relativistic heavy-ion collisions

The (σ, ω) model in the mean-field approximation where the meson fields are treated classically, describes much of observed nuclear structure and has been employed to describe the nuclear equation of state up to the quark-gluon phase transition. The acceleration of the meson sources, for example, in relativistic heavy-ion collisions, should result in bremsstrahlung-like radiation of the meson fields. The many mesons emitted serve to justify the use of classical meson fields. The slowing of the nuclei during the collision is modeled here as a smooth transition from initial to final velocity. Under ultra-relativistic conditions, vector radiation dominates. The angular distribution of energy flux shows a characteristic shape. It appears that if the vector meson field couples to the conserved baryon current, independent of the baryonic degrees of freedom, this mechanism will contribute to the radiation seen in relativistic heavy-ion collisions. The possible influence of the quark-gluon plasma is also considered.     

Equilibration in intermediate-energy heavy-ion collisions within a relativistic mean-field two-fluid model

A two-fluid model with meanσ- andω-fields is formulated for the treatment of heavy-ion collisions at incident energies around 1 GeV/u. In this energy range Fermi and Bose statistics for baryons and pions, respectively, cannot be replaced by Boltzmann statistics. The collisional coupling between the two fluids is formulated in terms of the effective nucleon-nucleon cross-sections in nuclear medium taking Pauli blocking into account. For two counterstreaming nuclear fluids the comparison of results obtained from our relativistic mean-field two-fluid model (RMF-TFM) and the relativistic Landau-Vlasov equation shows good agreement in the gross properties of the equilibration process.     

Nuclear shadowing effect in relativistic heavy-ion collisions

Two complementary approaches are used in studying the nuclear shadowing effect in heavy-ion collisions at SIS/GSI beam energies within a hadronic transport model. By analysing simultaneously the average transverse momentum of nucleons and pions in the reaction plane as a function of rapidity, the shadowing effect is revealed as an anticorrelation of the average transverse momentum distributions of nucleons and pions in semicentral and peripheral collisions. While by studying the azimuthal angle distribution of pions with respect to the reaction plane, the shadowing effect appears as an azimuthal anisotropy with a preferential emission of pions perpendicular to the reaction plane. The dependence of the nuclear shadowing effect on the impact parameter and the beam energy is also studied.     

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.