| In recent years, the equation of state of asymmetric nuclear matter, especially the high-density behavior of symmetry energy, plays a crucial role in understanding many problems and phenomena in nuclear physics and nuclear astrophysics. In heavy-ion collisions(HIC) at intermediate energies, dense nuclear matter can be formed with high asymmetry in neutron and proton numbers,and the HIC experiment is an unique tool to constrain the density-dependence of symmetry energy at high densities in laboratory. Since symmetry energy can not be measured directly, one has to use the transport model to simulate the dynamical process of HIC and compare the results with the experimental measurements to determine the density dependence of symmetry energy. Currently, one of the most important physical aims of the large-scale heavy ion facilities in China, the USA, Germany, Japan, etc, is to constrain the properties of asymmetric nuclear matter and the high-density behavior of symmetry energy.Within the framework of the isospin dependent Boltzmann–Uehling–Uhlenbeck(IBUU) transport model and the Ultra relativistic Quantum Molecular Dynamics(Ur QMD) transport model, various sensitive probes to the high-density behavior of symmetry energy in HIC at intermediate energies have been investigated and some valuable results are obtained. The model dependence of some sensitive probes to symmetry energy, i.e., the π-/π+ratio, the n/p ratio of free nucleons, the direct and elliptic flows of nucleons, and the neutron-proton differential transverse flow, has been explored and it is shown all these probes aremodel dependent. The effect of symmetry energy on the isospin-fractionation of nucleons has been studied. It is proposed that the normal or abnormal isospinfractionation of energetic nucleons can be considered as a qualitative probe to nuclear symmetry energy at suprasaturation densities. Then the effect of inmedium nucleon-nucleon(NN) elastic scattering cross sections on the π production in HIC at beam energies below the pion threshold has been investigated. It is shown that in the double ratio of π-/π+from neutron-rich and neutron-poor reaction systems(with the same mass number), the effects of the in-medium NN elastic scattering cross sections can be almost fully canceled out, while the effects of symmetry energy are kept almost completely. The effect of pion potential on the π-/π+ratio in HIC at beam energies around the pion threshold has been studied. Inclusion of the pion potential turns out to reduce the effect of the symmetry energy on the π-/π+ratio at beam energies below the pion threshold.The effects of ? resonance potential on the pre-equilibrium free n/p ratio and the π-/π+ratio have also been investigated. The results show that the effect of the ? resonance potential on the n/p ratio of pre-equilibrium emitted nucleons is invisible, and in HIC at intermediate energies, to avoid the problem of nonconservation of energy on ? or π productions, one can replace the ? potential by the nucleon isoscalar potential especially when a soft symmetry energy is employed. |
