Narrow Results

The dielectric function of nuclear matter excited by ρNN tensor coupling has been studied in the framework of finite temperature field theory. The induced current mechanism has been introduced to explain the three extrema on the dielectric function curve, of which one is in the space-like region and the other two are in the time-like region. It points out that the tensor coupling contributes much more large amplitude than the vector coupling and plays a more important role on the time-like region compared with its effect on the space-like region.

We consider the parity-violating two-pion-exchange potential obtained from the covari-ant formalism in the past and the state-of-the-art effective field theory approach. We discuss the behavior of the potential in coordinate space and its application to the parity-violating asymmetry in at threshold.

The effects of charge independence breaking of nucleon–nucleon interaction on the bulk properties of nuclear matter are investigated. Our results indicate that at high densities, the inclusion of charge dependence in the nucleon–nucleon potential affects the bulk properties of nuclear matter. However, at low densities, this effect is not considerable. It is seen that the change of our results for the nuclear matter calculations due to the breaking of the charge independence increases by increasing density. It is shown that the energy contribution of the ¹S₀ channel is sensitive to considering the charge dependence in the nucleon–nucleon interaction. It is indicated that the effects of charge independence breaking on the calculated equation of state of nuclear matter can be ignored.

The production of vector mesons in collisions between nucleons is studied in order to address a variety of issues concerning nucleon-nucleon interaction, reaction mechanism and properties of baryons. These studies are summarized with emphasis on the most recent experiments at the Time-of-Flight spectrometer TOF and results obtained at the COoler SYnchrotron COSY in Jülich. While currently the open questions regarding the so-called OZI violation, its relation to the meson exchange picture and the relative importance of contributions to the production mechanism from various channels within this formalism are still unresolved, the present-day experiments hold the potential to clarify the situation greatly. Possible extensions of the experimental program on vector mesons using 4π detection techniques for charged as well as neutral particles, in particular π⁰, are discussed.

It is argued that Weinberg's approach to the nucleon–nucleon (NN) interaction problem within effective field theory provides a consistent power counting for renormalized diagrams. Within this scheme the NN potential is organized as an expansion in terms of small quantities like small external momenta and the pion mass (divided by the characteristic large scale of the effective theory). Physical observables to any given order in these small quantities are calculated from the solutions of the Lippmann–Schwinger (or Schrödinger) equation.

In the no-core shell model formalism we compute effective one- and two-body operators, using the Lee–Suzuki procedure within the two-body cluster approximation. We evaluate the validity of the latter through calculations in reduced model spaces. In particular, we test the results for the two-body system and find that indeed the effective operators in the reduced space reproduce the expectation values or transition strengths computed in the full space. On the other hand, the renormalization for operators in the case of ⁶Li is very weak, suggesting the need for higher-body clusters in computing the effective interaction.

Pseudospin symmetry is an approximate relativistic symmetry of the nucleus as demonstrated by experimental data. This symmetry follows from the fact that the vector and scalar potentials of nucleons moving in a relativistic mean field are approximately equal in magnitude and opposite in sign. QCD sum rules in nuclear matter support this conclusion. Such an observation suggests a fundamental reason for pseudospin symmetry. We review the status of pseudospin symmetry conservation in the nucleon–nucleon interaction.

A Variational Monte Carlo method (VMC) is described for the evaluation of the ground state properties of nuclear matter. Equilibrium properties of symmetric nuclear matter and neutron matter are calculated by the described VMC method. The Urbana ν₁₄ potential is used for the nucleon–nucleon interactions in the calculations. Three- and more-body interactions are included as a density dependent potential term. Total, kinetic and potential energies per particle are obtained for nuclear and neutron matter. Pressure values of nuclear and neutron matter are also calculated at various densities. The binding energy of nuclear matter is found to be -16.06 MeV at a saturation density of 0.16 fm^-3. The results obtained are in good agreement with those obtained by various authors with different potentials and techniques.

The symmetric nuclear and neutron matter equation of state at finite temperature are calculated in the frame of the Thomas-Fermi approximation using the effective nucleon-nucleon interaction of Myers and Swiatecki. By introducing an effective mass in distribution function as a variational parameter, the effect of temperature on pressure, entropy, specific heat capacity, incompressibility and binding energy is discussed. A critical temperature of 17.2 MeV and a critical exponent of 0.32 for symmetric nuclear matter is found and we find that there is no phase transition in the neutron matter system. The results of calculations are in good agreement with experimental prediction and other theoretical results.

Three-nucleon systems are considered assuming the neutrons and protons to be distinguishable particles. The configuration space Faddeev equations are exploited to calculate ground state energies of ³H and ³He nuclei within an $s$-wave approach applying the Malfliet–Tjon, Tamagaki G3RS and Afnan–Tang ATS3 NN potentials. We modify the potentials by scaling strength parameters to define $nn$, $pp$ and $np$ singlet components. The scaling parameters are fixed to reproduce experimental scattering lengths. The charge symmetry breaking energy is numerically evaluated. The relation between $nn$, $pp$ and $np$ singlet potentials is discussed.

Finite, bound, many-body systems, where the interaction operator, $V=\sum v_{ij}$, is local and happens to strongly dominate the kinetic energy operator, $T=\sum t_i$, display a classical limit from diagonalizing $V$ alone, hence a clear picture of interparticle correlations, such as steric blocking emerges. This limit exhibits intrinsic symmetries and also fluctuations from mass formulae. This work investigates how such emergent symmetries and fluctuations might be extrapolated to physical situations where $T$ is reinstated.

We present an exploratory study consisting in the formulation of a relativistic quantum mechanics to describe the few-nucleon system at low energy, starting from the quantum field theoretical chiral Lagrangian involving pions and nucleons. To this aim we construct a Bakamjian-Thomas mass operator and perform a truncation of the Fock space which respects at each stage the relativistic covariance. Such truncation is justified, at sufficiently low energy, in the framework of a systematic chiral expansion. As an illustration we discuss the low-energy phaseshifts of the nucleon-nucleon and pion-nucleon scattering.

The theory of nuclear forces has made great progress since the turn of the millenium using the framework of chiral effective field theory (ChEFT). The advantage of this approach, which was originally proposed by Weinberg, is that it has a firm basis in quantum-chromodynamics and allows for quantitative calculations. Moreover, this theory generates two-nucleon forces (2NF) and many-body forces on an equal footing and provides an explanation for the empirically known fact that 2NF ≫ 3NF ≫ 4NF. I will present the recent advances in more detail and put them into historical context. In addition, I will also provide a critical evaluation of the progress made including a discussion of the limitations of the ChEFT approach.

The nucleon-nucleon interactions in finite nucleus are systematically investigated within the density-dependent relativistic Hartree-Fock (DDRHF) theory by taking ¹³²Sn as the representative. It is found that the neutron-neutron interactions in ¹³²Sn represent strong spin dependence, and such strong spin dependence is mainly determined by the Fock contributions in the isoscalar channels, i.e., the σ- and ω- couplings, which indicates that there may exist distinct tensor effects in the isoscalar channels as well as the isovector (ρ- and π) ones.

The nucleon-nucleon interaction is investigated by using the ImQMD model with the three sets of parameters IQ1, IQ2 and IQ3 in which the corresponding incompressibility coefficients of nuclear matter are different. Fusion excitation function and the charge distribution of fragments are calculated for reaction systems ⁴⁰Ca+⁴⁰Ca at different incident energies. It is found that obvious differences in the charge distribution were observed at the energy region 10-25AMeV by adopting the three sets of parameters, while the results were close to each other at energy region of 30-45A MeV for the reaction system. It indicates that the Fermi energy region is a sensitive energy region to explore the N-N interaction. The fragment multiplicity spectrum for ²³⁸U+¹⁹⁷Au at 15A MeV are reproduced by the ImQMD model with the set of parameter IQ3. It is concluded that charge distribution of the fragments and the fragment multiplicity spectrum are good observables for studying N-N interaction, and IQ3 is a suitable set of parameters for the ImQMD model.

We treat explicitly Δ (1232) isobar degrees of freedom using bare nucleonnucleon interaction, which can be the origin of the three-body forces via the pion exchange. We adopt the Argonne delta model potential (AV28) and study the explicit role of Δ in light nuclei. It is surprising that the additional Δ states generate strong tensor correlations from the transitions between N and Δ states, and change various matrix elements from the results with only the nucleon space.