Isospin In Heavy Ion Collisions And K Production At Near Threshold

The isospin effect and K production in intermediate and high energy Heavy Ion Collisions (HICs) are hot topics in the nuclear physics. Based on the Isospin-dependent Quantum Molecular Dyanmics (IQMD) model and self-consistent Relativistic Boltzmann-Uehling-Uhlenbeck (RBUU) model,we have studied them and obtained some interesting results.As for the study of isospin in intermediate energy HICs,we've investigated how both stength (Q) and density dependence of symmetry potential (SP) affect many measurable observables,such as the yield,phase-space,and isospin distributions of fragments,as well as the correlations between Intermediate-Mass-Fragment (IMF) multiplicity N and Charged-Particle multiplicity N,,Light-Charged-Particle (LCP) multiplicity N,and Neutron multiplicity N,. The aim is to find sensitive observables specifically for strength and density dependence of SP. Our results show:1),the rapidity distribution of differential neutron-proton counting (N) from more neutron-rich reactions can be favorable to extract the information of SP. 2),the correlation between N and Nc depends on the density dependence of SP more obviously. 3),the rapidity and phase-space distributions of Ny of emitted nucleons,LCP,and IMF show their different dependence on both Cs and density dependence of SP at different rapidity and phase-space regions,these results afford valueable observables,which can be used to extract information of strength and density dependence of SP. In the study of isospin-dependent nucleon-nucleon elastic cross sections in nuclear medium we find the medium correction of nucleon-nucleon scattering cross sections is also isospin dependent,cr depends on the baryon density weakly and cr i \ depends on the baryon density significantly. And based on these findings,we further study the dependence of nuclear stopping on system size,initial N/Z,SP and the medium correction of two-body cross sections. It's found that the effect of initial N/Z ratio and isospin SP on stopping is weak,and the excitation function of stopping depends on the form of medium correction of two-body cross sections and the equation of state of nuclear matter strongly. For probing the isospin dependence of the medium correction of two-body cross sections,the results show that the behavior of the excitation function of stopping for medium size nuclear collisions can provide clearer information than that for heavier systems. We have further studied equilibration with respect to isospin degree of freedom and proposed that the neutron-proton differential rapidity distribution is a sensitive probe to the energy dependence of the degree of equilibration:there exists memory effect in multifragmentation process;the degree of equilibration at E=400A MeVQingfeng Li,Isospin in HICs and K production at near thresholdis higher than that at EIOOA MeV;the average N/Z ratio of IMF reduces largely as beam energy increases from 100A MeV to 400A MeV.As for the K production in high energy HICs,we firstly study K+ production. The results show that the kaon flow is sensitive to both the kaon-nucleon sigma term (S and the equation of state of nuclear matter. The collective flows of both nucleons and K+ mesons need a "soft" EoS with compressibility K?00-300 MeV,and 2=200-400 MeV seems suitable to explain the measured K+ flow. We further find that anti-flow appears at certain large impact parameter,which seems to be more sensitive to 2 and might provide more definite information of the magnitude of 2. The absorption effect of K- mesons is also studied,we find although there exists strong K- absorption,K- spectrum can still contain the messages of AT" in nuclear medium. Our investigations show the ratio of the yield of K+ to K depends on the size of the nuclear systems,which is mainly caused by the absorption effect of the K in the nuclear medium. By detailed studies on both mean-absorptions path of the K in different nuclear systems and the individual effect of each of elementary production channels,we find both K - production and K -absorption eff...