R. Machleidt

Momentum Space Obep, Two Nucleon and Nuclear Matter Data

Abstract Several momentum-space OBEP are investigated regarding their results for the two-nucleon and nuclear matter system. The large discrepancies occurring in nuclear matter binding are traced back mainly to the different descriptions of NN data. The influence of relativistic effects and different scattering equations is discussed and turns out to be small.

An OBE model for the two-nucleon problem based on non-covariant perturbation theory☆

Abstract The NN scattering data and the deuteron properties are calculated within an OBE scheme based upon the treatment of a field theoretical Hamiltonian within old fashioned perturbation theory. The structure of the relevant equations is discussed in detail. The results show that one arrives at a consistent description of NN data — as consistent as with any other OBE model — so that the application of the same scheme to the calculation of meson current corrections to nuclear matter properties should be justified.

The Δ(1236) probability in the ground state of the nuclear many-body system

Abstract The probability of finding isobar configurations in the ground state of nuclear many-body systems is discussed for the deuteron, 16 O and infinite nuclear matter. A method is presented which allows the calculation of these probabilities taking into account effects of single-particle wave function and two-particle correlations in a self-consistent way. Momentum-space transition potentials are used that avoid the approximations which are necessary for r -space calculations, such as (a) π- and ρ-range propagators and (b) the static limit at the vertices. This yields considerably smaller values for the probability of one-Δ excitations, while the probability of exciting two Δ simultaneously is almost the same as in the approximate treatment.

The mass of a bound Δ-isobar☆

Abstract The properties of the Δ -isobar when bound in the nuclear medium are studied. The pionnucleon resonance nature of the Δ -isobar yields self-energy corrections, i.e., an energy- and medium-dependent mass and, above pion-production threshold, an energy- and medium-dependent width. Their importance on nucleon-nucleon scattering, the three-nucleon bound state and nuclear matter are discussed. Other effects arising from the resonance nature of the Δ -isobar are the retardation of the two-body interaction, three-nucleon forces and e.m. exchange currents.

On the effective σ-boson exchange in the relativistic Dirac-Brueckner approach to nuclear matter

Abstract A relativistic form of the Brueckner theory of nuclear matter is applied to an extended meson-exchange model for the NN-interaction which contains explicit 2π-and πϱ-exchange. This model avoids the effective σ-boson which is characteristic of the simplified meson exchange, as e.g. the one-boson-exchange (OBE) potential. It turns out that the relativistic saturation. effects found earlier within the OBE model are confirmed by the extended and more realistic model. In particular it is found that the relativistic effects caused by the explicit 2π-and πϱ-exchange are well simulated by the effective σ-boson of the OBE model.

An improved relativistic OBEP, two-nucleon and infinite nuclear matter data

Abstract A momentum-space OBEP consisting of π , η , ϱ , ω , φ , σ and δ -meson exchange is presented. Compared to a former paper (OBEP (I)) we refine the cut-off description and try to use more realistic values of the coupling constants. The quality of the resulting phase shift fit and the deuteron data is good ( χ 2 /datum = 2.7) and comparable to OBEP (I). The nuclear matter data calculated in exactly the same way as in the case of OBEP (I) are improved (now 11.2 MeV at k F = 1.47 fm −1 compared to 12.4 MeV at k F = 1.55 fm −1 for OBEP (I)), since the saturation density is consid lowered without losing too much binding.

Meson exchange corrections and properties of nuclear matter and neutron matter

Abstract A calculation of meson exchange corrections to the binding energy as function of the density is presented for nuclear matter and neutron matter. The framework is the application of non-covariant perturbation theory to a field theoretical Hamiltonian. Within a Brueckner-type approximation we restrict ourselves to the calculation of those meson exchange corrections which are due to one meson exchange and which produce no mass renormalization corrections. The results are reported in detail and the structure of the results is revealed. As a net effect, we find that our meson exchange corrections give a repulsion in nuclear matter yielding about 5 MeV less binding at the saturation point. For neutron matter, the effects are very small.

Role of the single-particle potential in nuclear matter calculations including mesonic and isobar degrees of freedom

Abstract Nuclear matter calculations in lowest-order Brueckner theory are performed starting from a realistic boson-exchange interaction, which includes isobar diagrams with π - and ρ -exchange at the NΔ vertices. Continuous single-particle spectra have been applied. Compared with former calculations using a free-particle spectrum, this choice yields more binding, keeping the density practically unchanged.

One-boson-exchange potential and the ground state of 16O

Abstract Two one-boson-exchange potentials (OBEP), which fit two-nucleon data and give reasonable results in nuclear matter, are tested in 16O. Ground state properties are calculated in the framework of the Brueckner-Hartree-Fock approach. The density dependence of the reaction matrix elements stemming from the Pauli operator and starting energy are carefully taken into account.

Influence of mesonic and nuclear selfenergies on the binding energy of nuclear matter

Abstract π - and ϱ -meson selfenergies are applied in nuclear matter calculations. It is shown that they lead to an additional attraction of nearly 10 MeV at nuclear matter density. However, this effect is almost exactly cancelled by nuclear selfenergy corrections.