NN Force Research Articles

Model independent determination of GEn from the [formula omitted] reaction

Electrodisintegration of vector polarized deuterium by longitudinally polarized electrons offers a possibility for measuring the neutron electric form factor G En. In principle, the electron asymmetry is essentially proportional to G En when quasi-free kinematics are employed. We investigate the model dependence of this result by employing several NN force models both with and without meson-exchange and isobar corrections. We find the asymmetry to be remarkably model independent near the neutron quasi-elastic peak.

Binding energies of the s-shell hypernuclei and the Λ well depth

Variational Monte Carlo calculations have been made for the s-shell hypernuclei; the well depth has been calculated variationally with the Fermi hypernetted chain method. A satisfactory description of all the relevant experimental Λ separation energies and also of the Λp scattering is obtained with reasonable phenomenological central two-pion exchange ΛN and ΛNN forces and strongly repulsive dispersive Λ NN forces. For the latter both spin-independent and spin-dependent forms were considered with the spin-dependent form preferred. The latter implies a smaller spin dependence of the ΛN force and contributes ⋍ 1 3 to the 0 +−1 + spin-flip splitting of the A = 4 hypernuclei. For the NN forces central Mafliet-Tjon potentials were mostly used.

Low-energy Lambda p scattering and p-shell hypernuclei

Phenomenological two-body central Lambda N potentials with possible state and spin dependence but without any hard or soft cores are used to analyse the existing low-energy Lambda p scattering data. The object is to see how far these free-state Lambda N potentials can account for the BLambda data of p-shell hypernuclei, Lambda 5He and Lambda binding to infinite nuclear matter when a suitable three-body Lambda NN force is added to these two-body potentials. The main aim of this work is to point out that by adding the Lambda NN force it is possible to choose effective Lambda N potentials whose two-body part is the same as the free Lambda N potential. The authors are able to find several such potentials. Out of many equivalent parameter sets accurate F/B data may place severe limitations on the l=1 part of the potentials, thus eliminating unphysical parameter sets. Therefore, a strong case is made for obtaining accurate F/B data.

Optimal channels for positive parity, strangeness −1 dibaryons

The question of the possible existence of positive-parity, strangeness −1 dibaryons lying below corresponding baryon-baryon thresholds is addressed in the context of a QCD-inspired potential model. One channel, with I = 1 2 , J = 3 , is found capable of supporting such a resonance. This prediction provides an independent test for the proposed physical mechanisms underlying recent descriptions of short-range NN forces in terms of quark interactions.

Theory of inclusive nuclear reactions: High-momentum component versus short-range correlation

Inclusive nuclear reactions are studied for various kinematical conditions. Even for the one-body operator, the one-nucleon knockout reaction mechanism is not always a dominant process in the presence of short-range nucleon-nucleon correlation. We give a general recipe for several inclusive processes so as to use the one-nucleon knockout reaction approximation with high-momentum components generated by the short-range NN force.

Test of proximity theorem for the nucleus-nucleus potential derived from a realistic NN force

The authors investigate the proximity scaling for the nuclear part of the nucleus-nucleus potential derived from the collision of two infinite nuclear matters flowing through each other. It is found that the proximity theorem is satisfied to a good extent at each value of the projectile energy per nucleon. They simplify the interaction potential between different pairs of nuclei by the product of a geometrical factor, dependent on the radii of the interacting nuclei, and a gaussian distribution whose parameters depend on the projectile energy per nucleon.

Nuclear binding and the skyrme model

A serious shortcoming of the Skyrme model is that it appears to predict for the NN interaction no attraction at medium range. We show that the symmetric quartic term, in powers of derivatives of the meson fields, not considered in the original Skyrme model, provides attractive NN forces. We argue that incorporation of vector mesons, especially the ω meson, along with this new quartic term into the Skyrme model can lead to a realistic description of the NN interaction.

Analysis of p-shell hypernuclei with ΛNN forces and the spin-state dependence of the ΛN effective interaction

From a chi 2 fit to the Lambda binding energy data of p-shell hypernuclei the authors have identified that a zero-range three-body Lambda NN force plays a crucial role in reducing the spin-state dependence of the central charge-independent two-body effective Lambda N potential of Skyrme-type, gaussian, Yukawa and exponential shapes. The exchange contribution of the three-body Lambda NN force to the interaction energy, either ignored altogether or not treated adequately in earlier analyses, has been shown to play an important role. For a Skyrme-type Lambda N potential found from the analysis of p-shell hypernuclei, the authors get a value of 28.52 MeV for Lambda binding to infinite nuclear matter and also Lambda binding in Lambda 5He turns out to be in good agreement with experiment. But the other shapes with reasonable values of the range parameter fail to account for Lambda binding to infinite nuclear matter and BLambda of Lambda 5He even qualitatively. The non-central Lambda N potentials (spin-orbit and tensor force) along with the low spin-state dependence of central two-body charge-independent Lambda N potentials fail to account for the data even qualitatively.

Parity nonconserving spin rotation in weak neutron-proton scattering

Parity nonconservation in weak n−p scattering is studied at threshold wehre neutron spin rotation might occur. We use the DDH weak NN force and a separable form with S, P and D partial waves in the strong NN interaction. The contribution of various spin-isospin components to the weak NN scattering amplitude is evaluated and the spin rotation angle is found to be −2.35 × 10 −9 rad/cm. The sources of possible errors are indicated.

Heavy-ion optical potentials at finite temperature calculated using a complex effective interaction derived from a realistic force

Hot density distributions of heavy ions generated by a modified Thomas-Fermi calculation at finite temperature are used to calculate the optical potential using the double-folding method and a complex effective energy and density-dependent interaction deduced from a realistic NN (Reid soft-core) force in nuclear matter. The real and the imaginary part of the optical potential become more attractive when the temperature increases.

Quark model and the nucleon-nucleon interaction

The short and the long range part of the nucleon-nucleon interaction for the 3S and 1S partial waves are discussed starting from the quark model. It is shown that the use of the Born-Oppenheimer approximation for the calculation of the nucleon-nucleon interaction is not sufficient. At short distances quark and gluon exchange describes the interaction. Nonlocalities of NN force and of the reduced nucleon-nucleon mass have to be taken into account. It is shown that at short distances the spatial [42] symmetry of the six quarks is very essential for the understanding of the short range repulsion. At large distances meson exchange is used to describe the interaction. To guarantee asymptotic freedom, the mesons are not allowed to couple to the quarks inside the bag. This is taken care of by assuming a r dependent coupling constant g πqq (r) = C π r 3 for the π and σ meson exchange. The two parameters are adjusted to the known meson nucleon coupling constants at zero momentum transfer. The parameters of the model are adjusted to the nucleon and Δ mass, to the root mean square radius of the proton including the π-meson cloud, to the pion-nucleon and the σ-nucleon coupling constants and to the σ mass m σ =520 MeV. The Resonating Group Method is used to solve the nucleon-nucleon scattering problem and to calculate the 3S and 1S phase shifts. They agree surprisingly well with the data.

Exact finite range DWBA calculations for heavy-ion induced nuclear reactions

Nuclear cluster radioactivity is investigated using microscopic potentials in the framework of the Wentzel–Kramers–Brillouin approximation of quantum tunneling by considering the Bohr–Sommerfeld quantization condition. The microscopic cluster–daughter potential is numerically constructed in the well-established double-folding model. A realistic M3Y-Paris NN interaction with the finite-range exchange part as well as the ordinary zero-range exchange NN force is considered in the present work. The influence of nuclear deformations on the cluster decay half-lives is investigated. Based on the available experimental data, the cluster preformation factors are extracted from the calculated and the measured half lives of cluster radioactivity. Some useful predictions of cluster emission half-lives are made for emissions of known clusters from possible candidates, which may guide future experiments.

207Pb(p, p') measurements at 135 and 61 MeV, the effective interaction and core polarization

Data at Einp = 61 and 135 MeV for neutron-hole transitions are used to study the proton-neutron part of the assumed NN force. Collective core-polarization strengths ( A L ) from the present fits at 135 MeV are consistent with those extracted from the (e, e') reaction for two L = 2 transitions and one L = 4 transition. This is not the case at 61 MeV where the A L values needed to fit the (p, p') data are much smaller for transitions to the f 5 2 and f 7 2 hole states. A fully microscopic DWBA fit is successful for the L = 3 transition to the doublet at 2.64 MeV of excitation at E p = 61 MeV, but fails at 135 MeV; a fully microscopic DWIA calculation provides a reasonable fit to the data at 135 MeV.

Effects of NN and N Tensor Forces on -Separation Energy of 5He

The Λ5He wave function is solved variationally with a realistic NN potential and a ΛN potential including a tensor force. The NN and ΛN tensor forces reduce the Λ-separation energy BΛ for Λ5He by about 2.0 and 1.6 MeV, respectively. The BΛ-value calculated here is 2.4 MeV and is not larger than the experimental one. A main part of the overbinding problem in Λ5He is resolved by introducing the NN and ΛN tensor forces.

Model nuclear matter calculations with a new fermion lowest order constrained variational method

We extend to deal with fennions a method of lowest order constrained variation for calculating the ground-state energy of dense systems, advanced previously by the present authors and shown to give excellent agreement for the Bethe homework problem. Using one state-independent correlation function in the new formalism, results with four different potentials that model various aspects of the real NN force are shown to be in excellent agreement with the other results available from various higher-order approximations, for densities up to and beyond nuclear matter density. The agreement is clearly superior to that obtained with any other lowest order approximation. When state dependence is introduced into the two-body correlation function in the new method, we find a considerable lowering of the energy in each case. It is suggested that this is a real effect which would be replicated in the other higher-order approximations, were they also extended to deal with state-dependent correlations.

Pion-deuteron elastic scattering in a relativistic three-body theory

Pion-deuteron elastic scattering in the region of the 3, 3 7π N resonance is considered in a relativistic three-particle theory. All spin and isospin effects are retained, as are all partial waves which make significant contribution to the scattering. The present numerical results are based on the inclusion of the P 33πN channel only, and on the neglect of the NN force in intermediate states. Pion rescattering effects are found to amount to about 20% contributions in cross sections. Sensitivity to the parametrization of the P 33 πN amplitude (including its off-energy-shell behavior), to the deuteron wave function, to the use of a nonrelativistic linearized propagator, and to the freezing of the deuteron by taking the nucleons to be infinitely massive is examined.

Charge asymmetry and neutron-neutron scattering

The uncertainty in the subtraction of electromagnetic effects from S-wave proton-proton scattering potentials is studied in two models using unitarily transformed potentials. Restrictions on these models caused by theoretical, off-shell and deuteron constraints are imposed. The probable uncertainty of physical interest is found to be small. It is typically only a few parts per thousand of the one-pion exchange potential. It has relatively little effect on Coulomb displacement energies. Phenomenological charge-symmetry-breaking potentials are constructed which both fit the experimental neutron-neutron (nn) scattering length (−16.4 fm). and give roughly the needed Coulomb displacement energies. These phenomenological potentials are found to contain a short-range repulsion and a strong long-range attraction for a neutron pair. The need for experimental information on the shape and state dependence of the nn force is emphasized. These can probably be obtained from precise nn scattering measurements.

Deuteron electromagnetic structure at large momentum transfer

Pursuing the idea that exchange currents should dominate the very large momentum transfer behavior of the deuteron electromagnetic form factors, we evaluate the $\ensuremath{\pi}\ensuremath{\rho}$ and $\ensuremath{\sigma}\ensuremath{\omega}$ exchange contributions. Taking the meson-nucleon coupling constants from one-boson exchange calculations of the $\mathrm{NN}$ force and the photon-meson coupling strengths from a relativistic quark model, we find that the form factors begin to "flatten out" at momentum transfers ${q}^{2}\ensuremath{\gtrsim}50$ ${\mathrm{fm}}^{\ensuremath{-}2}$ and are an order of magnitude greater than the usual impulse prediction at ${q}^{2}\ensuremath{\approx}100$ ${\mathrm{fm}}^{\ensuremath{-}2}$.NUCLEAR STRUCTURE $^{2}\mathrm{H}$; calculated exchange current effects in large momentum transfer electromagnetic form factors.

The nucleon-nucleon interaction and optimal relativity

The iterations of the Blankenbecler-Sugar equation are discussed and phase shifts are calculated for superpositions of one-boson-exchange interactions. A comparison with results obtained with the Bethe-Salpeter equation shows that an essentially quantitative agreement is achieved. The relevance of these calculations is discussed in relation to current techniques employed in the derivation of two-pion-exchange contributions to the NN force.

Short-range correlations and parity impurities

Two problems arising in calculating parity-violating effects in nuclei are discussed: the lack of reliable information on (i) weak parity-violating NN potentials and (ii) strong NN potentials at short relative distances and their influence on nuclear structure (short-range correlations). It is shown how to compute the latter reliably with the result that the parity admixtures depend strongly on the nucleon-nucleon potentials used. From this we conclude that the determination of the weak NN forces cannot be separated from the determination of the short-range behaviour of strong NN potentials.