Evolution of baryon-free matter produced in relativistic heavy-ion collisions

A three-fluid hydrodynamic model is introduced for simulating heavy-ion collisions at incident energies between a few and about 200 A GeV. In addition to the two baryon-rich fluids of two-fluid models, a new model incorporates a third, baryon-free (i.e., with zero net baryonic charge), fluid, which is created in the midrapidity region. Its evolution is delayed due to a formation time τ, during which the baryon-free fluid neither thermalizes nor interacts with the baryon-rich fluids. After formation, it thermalizes and starts to interact with the baryon-rich fluids. It is found that, for τ=0, the interaction strongly affects the baryon-free fluid. However, at reasonable finite formation time, τ ? 1 fm/c, the effect of this interaction turns out to be substantially reduced, although still noticeable. Baryonic observables are only slightly affected by the interaction with the baryon-free fluid.     

Transverse-momentum spectra of hadrons produced in central heavy-ion collisions

In-medium effects of transverse-mass distributions of quarks and gluons are considered assuming a possible local equilibrium for colorless quark objects like mesons and baryons created in central AA collisions. It is shown that the average square of the transverse momentum for these partons grows and then saturates when the initial energy increases. Within the quark-gluon string model, it leads to an energy dependence of hadron transverse-mass spectra which is similar to that observed in heavy-ion collisions. A comparison with other scenarios is given. The text was submitted by the authors in English.     

Back-to-back correlations of high-p(T) hadrons in relativistic heavy-ion collisions

We investigate the suppression factor and the azimuthal correlation function for high p(T) hadrons in central Au+Au collisions at sqrt[s(NN)]=200 GeV by using a dynamical model in which hydrodynamics is combined with explicitly traveling jets. We study the effects of parton energy loss in a hot medium, intrinsic k(T) of partons in a nucleus, and p (perpendicular) broadening of jets on the back-to-back correlations of high p(T) hadrons. Parton energy loss is found to be a dominant effect on the reduction of the awayside peaks in the correlation function.     

Formation of disoriented chiral condensates in relativistic heavy-ion collisions

We present a brief review of the subject of disoriented chiral condensates (DCC). We describe the conventional scenarios for the formation of DCC which have been proposed in the literature. Observable signals, such as fluctuations in neutral to charged pion ratio, are discussed. We then discuss a novel scenario for DCC formation, recently proposed by us, where the entire region of hot partons can get converted into a single large DCC. Our arguments suggest that formation of such large DCC is unlikely in the collision of heavy nuclei, and ultra-high energy hadronic collisions may be better suited for this.     

Mass spectrum of the dimuons produced in relativistic heavy-ion collisions

The mass spectrum of dimuons produced from the matter in the central region of rapidity in ultra-relativistic heavy-ion collisions is calculated in accordance with Bjorken’s recently proposed model for relativistic heavy-ion collisions. The matter in this central region is assumed to consist of a deconfined quark-gluon plasma phase and a pionized phase. Distinct enhancements of the dimuon mass spectrum below 500 MeV, due to the quark-gluon phase, are predicted for a deconfinement phase-transition temperatureT _c<200 MeV.     

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.     

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.     

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.     

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).     

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.     

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     

Formation and signature of quark matter in relativistic ion collisions

Exclusively using experimental information on particle production in nucleon-nucleon interactions, this paper attempts to demonstrate that: (i) the characteristics of relativistic collisions between heavy nuclei are determined by quark physics, not conventional nuclear physics; (ii) the formation of quark matter in such collisions can be observed experimentally via large multiplicities, copious production of photons (not from π decay), the anomalous strangeness, charm and baryon number structure of the events, and appearance of structure in the rapidity distribution; (iii) the formation of quark matter has actually been observed in high energy cosmic ray interactions. We show that the 100 TeV threshold for the appearance of anomalous interactions reflects the transition from nucleonic to quark structure of the nucleus. Observed anomalies match the signatures of quark matter formation in (ii); (iv) our results imply the abundant presence of heavy nuclei, e.g. Fe, in the high energy cosmic ray spectrum. Cosmic ray interactions above 100 TeV can eventually be used to study the vacuum structure of quantum chromodynamics and the disappearance of spontaneous symmetry breaking due to the restoration of symmetry at high matter densities.     

On the transverse momentum distribution of strange hadrons produced in relativistic heavy ion collisions

Particles with strange quark content produced in the system 1.93A·GeV⁵⁸Ni on⁵⁸Ni have been investigated at GSI Darmstadt with the FOPI detector system. The correlation of these produced particles was analyzed with respect to the reaction plane. baryons exhibit a very pronounced sideward flow pattern which is qualitatively similar to the proton flow. However, the kaon (K ⁺,K _s ⁰ ) flow patterns are significantly different from that of the protons, and their form may be useful to restrict theoretical models on the form of the kaon potential in the nuclear medium.The authors would like to thank G. Q. Li for helpful correspondence.     

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.