Zheng Yu-Ming

Differential Directed Flow of K+ Meson within Covariant Kaon Dynamics

The collective flow of positive charged kaons in heavy ion reactions at SIS energy is investigated within the frame of covariant kaon dynamics. The theoretical results calculated by using quantum molecular dynamics show that the rapidity distribution of K+ mesons is more sensitive to the nuclear equation of state than the differential directed flow. The contribution of various K+ production reaction channels to the rapidity distribution of the K+ is also analysed in detail. The results indicate that the rapidity distribution of K+ mesons is mainly from the contribution of the N-Δ and N-N channel. This means that the delta resonance state plays a predominantly important role for the K+ subthreshold production.

Transverse Flow of Kaons in Heavy-Ion Collisions

The transverse flow of positively charged kaons from heavy-ion collisions at intermediate energy is investigated within the framework of the quantum molecular dynamics model. The calculated results show that the experimental data are only consistent with those including the kaon mean-field potential from the chiral Lagrangian. This indicates that the transverse flow pattern of kaons is a useful probe of the kaon potential in a nuclear medium.

Influence of Chiral Mean Field on Kaon In-plane Flow in Heavy Ion Collisions

The influence of the chiral mean field on the in-plane flow in heavy ion collisions at SIS energy is investigated within covariant kaon dynamics. For the kaon mesons inside the nuclear medium a quasi-particle picture including scalar and vector fields is adopted and compared to the standard treatment with a static potential. It is confirmed that a Lorentz force from spatial component of the vector field provides an important contribution to the in-medium kaon dynamics and strongly counterbalances the influence of the vector potential on the in-plane flow. The calculated results show that the new FOPI data can be reasonably described using the Brown & Rho parametrization, which partly takes into account the correction of higher order contributions in the chiral expansion. This indicates that one can abstract the information on the kaon potential in a nuclear medium from the analysis of the in-plane flow.

Liquid Model of Phase Transition from Quark-Gluon Phase to Hadron Phase on Analogy of BCS Theory

A phenomenological model of the transition from quark-gluon phase to hadron phase is presented on the analogy of BCS theory. The massive current-quarks constitute the quark Cooper-pair, i.e., the hadron at , The order parameter of -pair takes a value in the range from 0.4 to . An experimental verification method of the present model in the heavy-ion collision is proposed.

Medium Suppression of In medium Nucleon-Nucleon Cross Sections Predicted with Various Microscopic Calculations

The nucleon-nucleon cross sections in the dense nuclear matter are microscopically calculated by using Dirac—Brueckner—Hartree—Fock (DBHF) approximation with different covariant representations of the T-matrix, i.e., complete pseudo-vector (CPV), pseudoscalar (PS) and pseudo-vector (PV) choices. Special attention is paid to the discrepancies among the cross sections calculated with these different T-matrix project choices. The results show that the medium suppression of the cross section given by DBHF in the CPV choice is not only smaller than those obtained in both PS and PV choices, but also smaller than the predictions with a nonrelativistic Brueckner—Hartree—Fock (BHF) method including three body force (3BF). The further analysis reveals that the influence of the different choices on the cross section in the DBHF approximation is mainly determined by the state of smaller total angular momentum due to the medium effect being strongly suppressed in the higher angular momentum.

Collective Flow of Λ Hyperons within Covariant Kaon Dynamics

The collective flow of Λ hyperons produced in association with positively charged kaon mesons in nuclear reactions at SIS energies is studied using the quantum molecular dynamics (QMD) model within covariant kaon dynamics. Our calculation indicates that both the directed and differential directed flows of Λs are almost in agreement with the experimental data. This suggest that the covariant kaon dynamics based on the chiral mean field approximation can not only explain the collective flow of kaon mesons, but also give reasonable results for the collective flow of Λ hyperons at SIS energies. The final-state interaction of Λ hyperons with dense nuclear matter enhances their directed flow and improves the agreement of their differential directed flow with the experimental data. The influence of the interaction on the Λ collective flow is more appreciable at large rapidity or transverse momentum region.

Observation of Influence of Nucleons' Motion on Kaons Collective Flow in Dense Medium

In order to extract the information of the momentum-dependent interaction of kaons under the extreme condition, the properties of the positively charged kaons produced in a heavy ion collision are studied via a simple model which has an invariable nucleons velocity. Our special attention is focused on the observation of the dependence of the kaons properties on the motion of nucleons in a hot and dense nuclear environment. Starting from two kinds of kaon quasiparticle models defined in transport theories for simulating heavy ion collisions, we calculate the effective mass and potential of the K+ s produced in the collisions and find that these properties not only depend closely on the velocity of nucleons but the dependence varies with kaons quasiparticle model. It is clearly shown that the motion of nucleons reduces the momentum of K+ s at a given rapidity and thus weakens the rapidity distribution of K+ s directed flow in realistic nuclear collisions.

Sigma Meson Production in Nuclear Reactions

The sigma meson production in p + 12C and p + 40Ca reactions at the incident energy Ep = 1.5 GeV is investigated within the Quantum Molecular Dynamics model. The simulation results indicate a distinctive A dependence of the sigma production, that is, the increase of A is followed by an increase of the production cross sections. We find that the σ meson production in proton-induced reactions is strongly medium-dependent, and the produced σ mesons decaying in a denser medium experience a stronger mass shift towards lower masses. This mass shift is an experimentally accessible observable in the final state pion pairs, which do not suffer from reabsorption by the surrounding nucleons. It is pointed out that the ratio of measured sigma cross sections as a function of the sigma invariant-mass from various reactions is a good probe to explore the existence of the σ meson in a dense nuclear environment.

Enhanced K+/π+ Ratio from Central Collisions Between Heavy Nuclei at 1 GeV/Nucleon*

We generalized the previous relativistic Vlasov-Uehling-Uhlenbeck model to be able to identify the isospin degree of freedom of particles and used it to simulate the particle production in Au + Au collisions at 1 GeV/nucleon. The momentum spectra of proton, π+ and K+ particles, the multiplicities of π+ and K+, and the K+/π+ ratio as functions of the number of participating nucleons are studied. The theoretical results agree very well with experimental data. We also analyzed the physical origin of the significant rising of K+/π+ ratio when peripheral collision goes to central collision.

Nuclear Liquid-Gas Phase Transition Shown in Multifragmentation*

The Incomplete-Fusion-Fragmentation Model has been used to reproduce nicely the experimental evidence of liquid-gas phase transition probed in the reaction of at . The analysis of the relative yields of decay modes for the projectile remnant as a function of shows clearly the competition and transformation processes of various decay modes of projectile remnant with decreasing of . The agreement between theoretical and experimental results for the analysis of relative yield of decay modes indicates that the experimental evidence of liquid-gas phase transition is composed of the decay mode transformations.