Elliptic flow and system size dependence of transition energies at intermediate energies

The elliptic flow for $Z\le2$ particles in heavy ion collisions at energies from several tens to several hundreds MeV per nucleon is investigated by means of transport model,i.e. a new version of the Improved Quantum Molecular Dynamics model (ImQMD05). In this model, a complete Skyrme potential energy density functional is employed. The influence of different effective interactions and medium corrections of nucleon-nucleon cross sections on the elliptic flow are studied. Our results show that a soft nuclear equation of state and incident energy dependent in-medium nucleon-nucleon cross sections are required for describing the excitation function of the elliptic flow at intermediate energies. The size dependence of transition energies for the elliptic flow at intermediate energies is also studied. The system size dependence of transition energies fits a power of system size with a exponent of 0.223.Comment: 12 pages, 4 figures, to be published in Phys.Rev.

Probing the symmetry energy and the degree of isospin equilibrium

The rapidity dependence of the single- and double- neutron to proton ratios of nucleon emission from isospin-asymmetric but mass-symmetric reactions Zr+Ru and Ru+Zr at energy range $100 \sim 800$ A MeV and impact parameter range $0\sim 8$ fm is investigated. The reaction system with isospin-asymmetry and mass-symmetry has the advantage of simultaneously showing up the dependence on the symmetry energy and the degree of the isospin equilibrium. We find that the beam energy- and the impact parameter dependence of the slope parameter of the double neutron to proton ratio ($F_D$) as function of rapidity are quite sensitive to the density dependence of symmetry energy, especially at energies $E_b\sim 400$ A MeV and reduced impact parameters around 0.5. Here the symmetry energy effect on the $F_D$ is enhanced, as compared to the single neutron to proton ratio. The degree of the equilibrium with respect to isospin (isospin mixing) in terms of the $F_D$ is addressed and its dependence on the symmetry energy is also discussed.Comment: 10 pages, 2 figure

In-medium NN cross sections determined from stopping and collective flow in intermediate-energy heavy-ion collisions

In-medium nucleon-nucleon scattering cross sections are explored by comparing results of quantum molecular dynamics simulations to data on stopping and on elliptic and directed flow in intermediate-energy heavy-ion collisions. The comparison points to in-medium cross sections which are suppressed at low energies but not at higher energies. Positive correlations are found between the degree of stopping and the magnitudes of elliptic and directed flows.Comment: 11 pages, 4 figures, to be published on PR

Re-visit the N/Z ratio of free nucleons from collisions of neutron -rich nuclei as a probe of EoS of asymmetric nuclear matter

The N/Z ratio of free nucleons from collisions of neutron-rich nuclei as a function of their momentum is studied by means of Isospin dependent Quantum Molecular Dynamics. We find that this ratio is not only sensitive to the form of the density dependence of the symmetry potential energy but also its strength determined by the symmetry energy coefficient. The uncertainties about the symmetry energy coefficient influence the accuracy of probing the density dependence of the symmetry energy by means of the N/Z ratio of free nucleons of neutron-rich nuclei.Comment: 15 pages, 6 figures, 2 tables. accepted by Commun. Theor. Phys. (Beijing, China

Production and rescattering of strange baryons at SPS energies in a transport model with hadron potentials

A mean-field potential version of the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model is used to investigate the production of strange baryons, especially the $\Lambda$s and $\overline{\Lambda}$s, from heavy ion collisions at SPS energies. It is found that, with the consideration of both formed and pre-formed hadron potentials in UrQMD, the transverse mass and longitudinal rapidity distributions of experimental data of both $\Lambda$s and $\overline{\Lambda}$s can be quantitatively explained fairly well. Our investigation also shows that both the production mechanism and the rescattering process of hadrons play important roles in the final yield of strange baryons.Comment: 15 pages, 7 figure

Transport model study of nuclear stopping in heavy ion collisions over an energy range from 0.09A GeV to 160A GeV

Nuclear stopping in the heavy ion collisions over a beam energy range from SIS, AGS up to SPS is studied in the framework of the modified UrQMD transport model, in which mean field potentials of both formed and "pre-formed" hadrons (from string fragmentation) and medium modified nucleon-nucleon elastic cross sections are considered. It is found that the nuclear stopping is influenced by both the stiffness of the equation of state and the medium modifications of nucleon-nucleon cross sections at SIS energies. At the high SPS energies, the two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood with the consideration of the "pre-formed" hadron potentials.Comment: 15 pages, 7 figure

Mass and Isotope Dependence of Limiting Temperatures for Hot Nuclei

The mass and isotope dependence of limiting temperatures for hot nuclei are investigated. The predicted mass dependence of limiting temperatures is in good agreement with data derived from the caloric curve data. The predicted isotope distribution of limiting temperatures appears to be a parabolic shape and its centroid is not located at the isotope on the $\beta$-stability line(T=0) but at neutron-rich side. Our study shows that the mass and isotope dependence of limiting temperatures depend on the interaction and the form of surface tension and its isopin dependence sensitively.Comment: 14 pages,11 figure