arXiv subjects

Bao-An Li

Publications and source records attributed to Bao-An Li.

At least 253 records · Page 14Linked to original sources

arXiv2001

Charged particle rapidity distributions at RHIC

Using a multiphase transport model (AMPT), which includes both initial partonic and final hadronic interactions, we study the rapidity distributions of charged particles such as protons, antiprotons, pions, and kaons in heavy ion collisions at RHIC. The theoretical results for the total charged particle multiplicity at midrapidity are consistent with those measured by the PHOBOS collaboration in central Au+Au collisions at $\sqrt s$ = 56 and 130 AGeV. We find that these hadronic observables are much more sensitive to the hadronic interactions than the partonic interactions.

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arXiv2000

Neutron-Proton Differential Flow as a Probe of Isospin-Dependence of Nuclear Equation of State

The neutron-proton differential flow is shown to be a very useful probe of the isospin-dependence of the nuclear equation of state (EOS). This novel approach utilizes constructively both the isospin fractionation and the nuclear collective flow as well as their sensitivities to the isospin-dependence of the nuclear EOS. It also avoids effectively uncertainties associated with other dynamical ingredients of heavy-ion reactions at intermediate energies.

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arXiv2000

J/psi suppression in ultrarelativistic nuclear collisions

Using a multiphase transport model, we study the relative importance of J/psi suppression mechanisms due to plasma screening, gluon scattering, and hadron absorption in heavy ion collisions at the Relativistic Heavy Ion Collider. We find that for collisions between heavy nuclei such as Au+Au, both plasma screening and gluon scattering are important. As a result, the effect due to absorption by hadrons becomes relatively minor. The final J/psi survival probability in these collisions is only a few percent. In the case of collisions between light nuclei such as S+S, the effect of plasma screening is, however, negligible in spite of the initial high parton density. The final J/psi survival probability thus remains appreciable after comparable absorption effects due to gluons and hadrons.

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arXiv1999

Uranium on uranium collisions at relativistic energies

Deformation and orientation effects on compression, elliptic flow and particle production in uranium on uranium collisions (UU) at relativistic energies are studied within the transport model ART. The density compression in tip-tip UU collisions is found to be about 30% higher and lasts approximately 50% longer than in body-body or spherical UU reactions. The body-body UU collisions have the unique feature that the nucleon elliptic flow is the highest in the most central collisions and remain a constant throughout the reaction. We point out that the tip-tip UU collisions are more probable to create the QGP at AGS and SPS energies while the body-body UU collisions are more useful for studying properties of the QGP at higher energies.

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arXiv1999

A Multi-Phase Transport model for nuclear collisions at RHIC

To study heavy ion collisions at energies available from the Relativistic Heavy Ion Collider, we have developed a multi-phase transport model that includes both initial partonic and final hadronic interactions. Specifically, the parton cascade model ZPC, which uses as input the parton distribution from the HIJING model, is extended to include the quark-gluon to hadronic matter transition and also final-state hadronic interactions based on the ART model. Predictions of the model for central Au on Au collisions at RHIC are reported.

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arXiv1999

Elliptic flow in heavy ion collisions near the balance energy

The proton elliptic flow in collisions of Ca on Ca at energies from 30 to 100 MeV/nucleon is studied in an isospin-dependent transport model. With increasing incident energy, the elliptic flow shows a transition from positive to negative flow. Its magnitude depends on both the nuclear equation of state (EOS) and the nucleon-nucleon scattering cross section. Different elliptic flows are obtained for a stiff EOS with free nucleon-nucleon cross sections and a soft EOS with reduced nucleon-nucleon cross sections, although both lead to vanishing in-plane transverse flow at the same balance energy. The study of both in-plane and elliptic flows at intermediate energies thus provides a means to extract simultaneously the information on the nuclear equation of state and the nucleon-nucleon scattering cross section in medium.

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arXiv1999

Directed flow of neutral strange particles at AGS

Directed flow of neutral strange particles in heavy ion collisions at AGS is studied in the ART transport model. Using a lambda mean-field potential which is 2/3 of that for a nucleon as predicted by the constituent quark model, lambdas are found to flow with protons but with a smaller flow parameter as observed in experiments. For kaons, their repulsive potential, which is calculated from the impulse approximation using the measured kaon-nucleon scattering length, leads to a smaller anti-flow than that shown in the preliminary E895 data. Implications of this discrepancy are discussed.

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arXiv1999

Kaon differential flow in relativistic heavy-ion collisions

Using a relativistic transport model, we study the azimuthal momentum asymmetry of kaons with fixed transverse momentum, i.e., the differential flow, in heavy-ion collisions at beam momentum of 6 GeV/c per nucleon, available from the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory (BNL). We find that in the absence of kaon potential the kaon differential flow is positive and increases with transverse momentum as that of nucleons. The repulsive kaon potential as predicted by theoretical models, however, reduces the kaon differetnial flow, changing it to negative for kaons with low momenta. Cancellation between the negative differential flow at low mementa and the positive one at high momenta is then responsible for the experimentally observed nearly vanishing in-plane transverse flow of kaons in heavy ion experiments.

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arXiv1999

Differential flow in heavy-ion collisions at balance energies

A strong differential transverse collective flow is predicted for the first time to occur in heavy-ion collisions at balance energies. We also give a novel explanation for the disappearance of the total transverse collective flow at the balance energies. It is further shown that the differential flow especially at high transverse momenta is a useful microscope capable of resolving the balance energy's dual sensitivity to both the nuclear equation of state and in-medium nucleon-nucleon cross sections in the reaction dynamics.

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arXiv1999

A transport model for heavy ion collisions at RHIC

To study heavy ion collisions at energies to be available from the Relativistic Heavy Ion Collider (RHIC), we have developed a transport model that includes both initial partonic and final hadronic interactions. Specifically, the parton cascade model ZPC, which uses as input the parton distribution from the HIJING model, is extended to include the quark-gluon to hadronic matter transition and also final-state hadronic interactions based on the ART model. Comparisons with SPS data and predictions for RHIC are reported.

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arXiv1999

Excitation function of nucleon and pion elliptic flow in relativistic heavy-ion collisions

Within a relativistic transport (ART) model for heavy-ion collisions, we show that the recently observed characteristic change from out-of-plane to in-plane elliptic flow of protons in mid-central Au+Au collisions as the incident energy increases is consistent with the calculated results using a stiff nuclear equation of state (K=380 MeV). We have also studied the elliptic flow of pions and the transverse momentum dependence of both the nucleon and pion elliptic flow in order to gain further insight about the collision dynamics.

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arXiv1998

Antikaon Production and Medium Effects in Heavy Ion Collisions at AGS

Antikaon production from heavy ion collisions at energies available from the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory is studied in a relativistic transport (ART) model. We include contributions from the baryon-baryon, meson-baryon, and meson-meson interactions. The final-state interaction of antikaons via both absorption and elastic scattering by nucleons and pions are also considered. To compare with presently available or future experimental data, we have calculated the antikaon rapidity and transverse momentum distributions as well as its collective flow. Medium effects on these observables due to mean field potentials have also been investigated. It is found that the ratio of antikaon transverse momentum spectrum to that of kaon and their transverse flow are most sensitive to the in-medium properties of kaons and antikaons.

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arXiv1998

Probing the softest region of nuclear equation of state

An attractive, energy-dependent mean-field potential for baryons is introduced in order to generate a soft region in the nuclear equation-of-state, as suggested by recent lattice QCD calculations of baryon free matter at finite temperature. Based on a hadronic transport model, we find that although this equation-of-state has negligible effects on the inclusive hadronic spectra, it leads to a minimum in the energy dependence of the transverse collective flow and a delayed expansion of the compressed matter. In particular, the transverse flow changes its direction as the colliding system passes through the softest region in the equation-of-state.

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arXiv1998

Isospin relaxation time in heavy-ion collisions at intermediate energies

Using an isospin-dependent transport model, we have studied the isospin and momentum relaxation times in the heavy residues formed in heavy-ion collisions at intermediate energies. It is found that only at incident energies below the Fermi energy, chemical or thermal equilibrium can be reached before dynamical instability is developed in the heavy residues. Also, the isospin relaxation time is shorter (longer) than that for momentum at beam energies lower (higher) than the Fermi energy.

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arXiv1997

Isospin Physics in Heavy-Ion Collisions at Intermediate Energies

In nuclear collisions induced by stable or radioactive neutron-rich nuclei a transient state of nuclear matter with an appreciable isospin asymmetry as well as thermal and compressional excitation can be created. This offers the possibility to study the properties of nuclear matter in the region between symmetric nuclear matter and pure neutron matter. In this review, we discuss recent theoretical studies of the equation of state of isospin-asymmetric nuclear matter and its relations to the properties of neutron stars and radioactive nuclei. Chemical and mechanical instabilities as well as the liquid-gas phase transition in asymmetric nuclear matter are investigated. The in-medium nucleon-nucleon cross sections at different isospin states are reviewed as they affect significantly the dynamics of heavy ion collisions induced by radioactive beams. We then discuss an isospin-dependent transport model, which includes different mean-field potentials and cross sections for the proton and neutron, and its application to these reactions. Furthermore, we review the comparisons between theoretical predictions and available experimental data. In particular, we discuss the study of nuclear stopping in terms of isospin equilibration, the dependence of nuclear collective flow and balance energy on the isospin-dependent nuclear equation of state and cross sections, the isospin dependence of total nuclear reaction cross sections, and the role of isospin in preequilibrium nucleon emissions and subthreshold pion production.

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arXiv1997

Excitation Functions of Stopping Power and Flow in Relativistic Heavy-Ion Collisions

Using a relativistic transport (ART) model, we study the stopping power, the formation of superdense hadronic matter as well as the strength of transverse and radial flow in central Au+Au collisions at beam momentum from 2 to 12 GeV/c per nucleon. We find that complete stopping is achieved in the whole beam momentum range. In particular, the proton rapidity distribution scaled by the beam rapidity is independent of the beam momentum, and this is in agreement with the experimental findings. Also, a large volume of superdense hadronic matter with a local energy density exceeding that expected for the transition of a hadronic matter to the quark-gluon plasma is formed in collisions at beam momenta greater than 8 GeV/c per nucleon. Furthermore, it is found that the transverse flow in these collisions is sensitive to the nuclear equation of state and decreases with increasing beam momentum. On the other hand, the radial flow is insensitive to the equation of state, and its strength increases with beam momentum.

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arXiv1997

Chemical and mechanical instabilities in hot, isospin-asymmetric nuclear matter

Using phenomenological equations of state, we study the chemical and mechanical instabilities in hot isospin-asymmetric nuclear matter. Both instabilities are found to depend strongly on the isospin asymmetry of the nuclear matter. For a chemically and mechanically stable asymmetric nuclear matter, a lower neutron excess is found in the liquid phase than in the gas phase. Furthermore, the neutron excess in the mixed phase at densities between that of the liquid and the gas phases is limited by certain maximum value. Also, for the nuclear equations of state considered here the boundary of chemical instability in the pressure-density plane is found to be more extended than that of mechanical instability. However, locations of these boundaries depend sensitively on both the bulk compressibility of nuclear matter and the density dependence of the symmetry energy.

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arXiv1997

Equation of state of asymmetric nuclear matter and collisions of neutron-rich nuclei

The ratio of pre-equilibrium neutrons to protons from collisions of neutron-rich nuclei is studied as a function of their kinetic energies. This ratio is found to be sensitive to the density dependence of the nuclear symmetry energy, but is independent of the compressibility of symmetric nuclear matter and the in-medium nucleon-nucleon cross sections. The experimental measurement of this ratio thus provides a novel means for determining the nuclear equation of state of asymmetric nuclear matter.

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