One-boson Exchange Model Research Articles

Unitary symmetry in baryon-baryon scattering

Motivated by a recent suggestion that an SU(3) selection rule may explain the narrow widths of some hypernuclear excitations in the Σ continuum, we re-examine the role of unitary symmetry in low energy baryon-baryon (BB) interactions. A direct examination of the BB scattering data indicates that SU(3) symmetry is broken, but leaves us with several alternative schemes for describing the mechanism of symmetry breaking. We study the one-boson exchange (OBE) model in some detail, and relate the usual spin-isospin representation of BB potentials/amplitudes to the quadratic and cubic SU(3) Casimir operators F 2 and G 3. For the long-range part of the BB interaction (pseudoscalar meson exchange), the G 3 amplitude is important, whereas the short-range part (quark-gluon or vector meson exchange) leads to the F 2 form. In the Nijmegen OBE model, which is very successful in reproducing the data, SU(3) symmetry is maintained at the level of coupling constants, and broken through the use of observed meson and baryon masses in the calculation of baryon-baryon potentials V BB. In spite of the sizable mass differences and the use of phenomenological hard cores at short distances in the OBE approach, certain SU(3) relations are still reasonably well satisfied. We also discuss other mechanisms of SU(3) breaking which involve an explicit hypercharge dependence of the BB amplitudes.

Decay of hypernuclei

The experimental and theoretical situation regarding the mesonic and nonmesonic decays of light hypernuclei is reviewed. Nuclear matter and shell model estimates for nonmesonic decays are compared within the one-boson exchange model approximation. The importance of strangeness exchange is explored as well as the spin-isospin dependence of the nonmesonic weak decays.

Isoscalar currents and nuclear magnetic moments

We investigate meson-exchange current effects on isoscalar nuclear magnetic moments taking a one-boson exchange model. For some cases we find appreciable contributions, though the results are very model dependent. We also discuss the connection between exchange currents and the calculation of magnetic moments in the relativistic σ-ω model.

Nucleon-nucleon interactions and quarks I: General considerations with numerical results in the 1S0 channel

In this paper, we discuss a general development of a quark-model description of nucleon-nucleon (NN) interactions, assuming that quarks are Dirac particles and nucleons are specified approximately by flavor SU(6) wave functions. The down (d) quark is differentiated from the up (u) quark allowing isospin symmetry violating effects. As a simple model calculation, we include effects due to one-gluon exchange (which dictates the short-range behavior of NN forces), one-pion exchange (which gives rise to the Yukawa tail in NN forces), single sigma-exchange (which mocks up to the intermediate attraction in an economical manner), and one-photon exchange (which contributes to isospin symmetry violations). To avoid introduction of additional shape parameters, we adopt a double-sphere geometry with an adjustable isolated nucleon radius. All quark-interchange effects are explicitly evaluated and Pauli blocking is incorporated. Numerical results in the 1 S 0 p− p, n− p, and n− n channels are described and discussed. In particular, it is found that the simple one-boson exchange model (at the quark level) provides a reasonable description of the 1 S 0 phase shifts for 10 MeV ≤ T CM ≤ 150 MeV with T CM the center-of-mass [CM] kinetic energy. It is also found that, with the nonzero up-down quark mass difference as determined by an overall fit to the observed baryon mass splittings, isospin-symmetry violating effects arising from quark interchange are of marginal numerical importance for a nucleon radius R of 0.8 fm and become fairly sizable for R = 1.0 fm. It turns out that, for R = 0.8 fm, the predicted a nn with a nn the low-energy scattering lengths in the 1 S 0 n- n channel is in good agreement with a recent experimental result of Gabioud et al. and the observed large charge dependent effect as given by | a np | − | a nn | can also be understood.

The Skyrmion Skyrmion interaction and the meson exchange model for the NN potential

The properties of nucleon-nucleon (NN) potentials derived from the Skyrme model are investigated compared to those of typical One-Boson-Exchange (OBE) models, For that purpose the NN as well as the nucleon-antinucleon (N¯N) potentials are evaluated for the isoscalar central channel, the isovector spin channel, and the isovector tensor channel. A comparison of NN and N¯N interactions allows a separation into terms with positive and negativeG-parity and therefore a detailed comparison with the properties of the OBE model. This analysis is performed using the classical Lagrangian of the Skyrme model and considering an extension of this Lagrangian by a term of sixth order, which has recently been proposed. The results indicate that the extended Lagrangian yields an interaction which is quite consistent with the OBE picture. The extended model, however, is much more sensitive to the values of the parameters and the approximations being used for the evaluation of the field for two baryons.

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

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.

Test of the nuclear matter G-matrix interaction in nuclear structure calculations

A nuclear matter G-matrix interaction, based on the one-boson exchange model for the nuclear force, has been tested in the description of collective excited states of nuclei. It is explicitly demonstrated that this interaction leads to an instability in heavier nuclei such as 208Pb when used as effective particle-hole interaction within the framework of the random-phase approximation. The necessity to include two-particle—two-hole processes in a consistent way is discussed in a schematic model.

-Nucleon Tensor Force in Hypertriton

The hypertriton ~H is the lightest and loosely bound hypernucleus and is of fundamental impor­ tiillce to get the knowledge of the A -nucleon interaction. Thereupon we examine the effect of A-N tensor force in the bound state of the hyper­ triton by solving the Faddeev equation in the coordinate .space, considering the fact that the neutron-proton tensor force plays an inportant role to make the realistic estimate of the binding energy in the hypertriton. The meson theoretical A-N potential due to one boson exchange (OBE) has been proposed by N agels et al. I) in order to reproduce the available nucleon-nucleon and hyperon-nucleon scattering data, where the latest ones are the model D and F. Since the tensor force of the model D is stronger than that of the model F, it is more effective to examine a local A-N potential including the tensor force with repulsive soft core based on this OBE model D potential, where the explicit AN-2:N coupling is not introduced. We previously estimated the A -separation energy BA of the hypertriton using the effective central A -N potential with three term Yukawa form including the repulsive soft core and the Reid soft core n-p potential including the tensor force 2 ) by solving the Faddeev equation in the coordinate space and obtained the BA value of 0.17 MeV. 3 ) It was then pointed out that the n-p tensor force is important for the bound state of the hypertriton. Gibson and Lehman re-evaluat­ ed the A -separation energy BA of ~H using the A-N separable central potential and also n-p separable potential including the tensor force by solving the Faddeev equation in the momentum space.') Their results for BA were 0.13 MeV

Conversion of hyperons in nuclear matter

The width Γ of the ∑N → ΛN conversion for ∑ hyperons in nuclear matter is calculated on the basis of the one-boson exchange model. It turns out that there is a substantial compensation of the contributions of different mesons to the conversion amplitude. As a result, Γ is reduced by roughly a factor of two as compared to the value from the exchange of only a pion. Without taking the Pauli principle into account, we obtain Γ = 15–25 MeV.

Electromagnetic properties of the deuteron in a relativistic one-boson exchange model

The deuteron electric electromagnetic form factors are studied in a quasi-potential framework, where relativistic and meson-exchange contributions are treated consistently. At moderate momentum transfer the corrections to the static approximation are found to be significantly less than estimates obtained in the conventional perturbational treatment. It is argued that this is due to the neglect of dynamical corrections to the wave function in the latter approach.

Meson exchange model for the nucleon-nucleon interaction

Nucleon-nucleon interactions obtained from several models for the field theoretic scattering amplitude are studied. The interaction includes contributions from one-pion and one-omega exchange and from two-pion exchange as calculated in a dispersion theory framework. The resulting interaction is regularized by a cut-off factor obtained by the eikonal approximation to multiple vector meson exchange processes. The Blankenbecler-Sugar equation is solved with the interaction and nucleon-nucleon scattering phase parameters are computed. For the best model good agreement with phenomenological phase parameters is achieved for physically reasonable values of the meson-nucleon coupling constants and the spectral functions needed for the evaluation of the two-pion exchange effects. The deuteron wave function is computed as are the deuteron charge and quadrupole form factors. The interaction is shown to have significantly weaker short-range repulsion than commonly found in local phenomenological potentials and in one-boson exchange models.

A Unified Model of the One-Boson- and Reggeon-Exchange Mechanism inN-NandN-$\bar{N}$ Interaction

This is an application of our extended-boson exchange amplitude to p-p and p-p scatter­ ing at high energies (10-'30GeV) and at forward angles with iti:$1(GeV/c)2• The amplitude is constructed such that it is reducible to the one-boson exchange amplitude in the low energy region, while it tends to the Reggeon-exchange amplitude at small angles and to a modified one-boson-exchange amplitude at large angles in the high energy region. We find that our amplitude is quantitatively adaptable to both data of the low and high energies with It I :$1 (GeV/c) 2• For the last decade many people have reported fair success m description of N-N and N-N interactions in terms of the one-boson-exchange modePl~ 6) in the low energy region (E$1Ge V) and the Regge pole model6l in the high energy region (E> lOGe V). In this situation we believe that unification of both models is quite worthwhile. The Veneziano model is one of trials for such unification. However some difficulties have been reported in its application to N-N and N-N problems.7l Recently we have proposed an extended-boson-exchange (EBE) amplitude for non-diffractive part of the N-N and N-N interactions. 8l The extended boson is described by a bilocal field representing a space-time extension of the boson. We have shown that, with an appropriate choice of a function for describing the space-time extension, the EBE amplitude can be reduced to the one-boson-exchange amplitude in the low energy region, while it tends to a Reggeon-exchange amplitude at forward angles and to a modified one-boson-exchange amplitude at large angles in the high energy region. With this property the EBE amplitude makes the unification of the one-boson-exchange model (OBEM) and the Regge pole model (RPM). This paper reports a result of quantitative application of the EBE amplitude to the N.N and N-N problems and shows that fits of the amplitude to the low and high energy data are satisfactory. We set up a model as follows. The whole amplitude for N-N and N-N scattering is divided into diffractive and non­ diffractive part. Possible contribution from the resonance formation in N-N scat­ tering is now classified into the diffractive part. The non-diffractive part is assumed to be a. sum of the one-n-, one-0-, one-w-, one-{5-, and one-!0- exchange amplitudes where we use notations w, i5 and lo for the extended ()), p and fo

An Infinite-One-Boson-Exchange Type Model for High-Energy Elastic Proton-Proton Scattering

For high-energy elastic proton-proton scattering we try an infinite one-boson-exchange (IOBE) type model as a generalization of the one-boson-exchange (OBE) model of strong interactions. Veneziano-like amplitudes are assumed for an infinite sum of one-particle-exchange amplitudes. In the present analysis we show that the coupling parameters required to reproduce experimental data at high energies TL = 5 ∼15 GeV are compatible with the corresponding ones of the OBE model obtained from low-energy analyses.

Multichannel calculations of hyperon-nucleon cross-sections and phase shifts

We compute the hyperon-nucleon cross-sections by taking care of the coupling between the ΛN and ΣN channels. For this purpose a multichannelND−1 method is used within the framework of the one-boson exchange model. The number of coupling constants that we need is reduced by symmetry arguments and the free parameters left are fixed by fitting the ΛN cross-section data. Good fits are obtained and the predictions of the model concerning the ΣN cross-sections are in good agreement with the experimental data. Furthermore a hyperon-nucleon phase-shift analysis is performed. No indication of a resonance is found; the Λp cross-section enhancement attributed sometimes to a3S1 Λp resonance seems to be accounted for by the coupling effect between ΛN and ΣN channels.

Coupled-channel effective-range analysis of hyperon-nucleon interactions

Scattering lengths and effective ranges of low-energy hyperon-nucleon scattering are computed from completely determined amplitudes obtained in a multichannelND−1 formalism within the framework of the one-boson exchange model. Comparison is made with results of different models based on coupled Schrodinger equations with various potentials and good agreement with scattering and hypernuclei data is observed.

Spin-Orbit Coupling forΛ−He4Scattering Using a One-Boson-Exchange Model for theΛNInteraction

The polarization and cross sections for $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ elastic scattering are calculated for $\ensuremath{\Lambda}$ incident energies 1-20 MeV. A $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ potential is constructed using a phenomenological $\ensuremath{\Lambda}N$ Gaussian central potential and the empirical shape for $^{4}\mathrm{He}$, and a spin-orbit potential obtained from a one-boson-exchange (OBE) model for the $\ensuremath{\Lambda}N$ interaction. As indicated recently by Downs, the OBE model for $\ensuremath{\Lambda}N$ forces leads to strong antisymmetric spin-orbit terms due to vector-meson exchange. This means that the $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ potential depends only on part of the $\ensuremath{\Lambda}N$ spin-orbit interaction, since it results from summing the latter interaction over the nucleon spin states. Parameters from three quite different OBE calculations are used to predict the $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ scattering, and these three sets of OBE parameters lead to remarkably similar predictions concerning the cross sections and polarization properties for this energy range. However, no $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ resonances are predicted, in contrast to recent Hartree-Fock calculations of $\ensuremath{\Lambda}\ensuremath{-}^{4}\mathrm{He}$ scattering. The polarization effects predicted are large, and experimental data concerning them would have interesting consequences for our knowledge of the origin of spin-orbit forces in the baryon-baryon system.

Two-pion exchange contribution to the nucleon-nucleon force and an effective σ-meson

The longest range contribution to the nucleon-nucleon force, due to two-pion exchange with a virtual N 33 ∗ isobar in the intermediate states, is calculated. The terms with only nucleons in the intermediate states is added to this, and the potentials which would result from σ ( J P = 0 +, T = 0) and σ′( J P = 0 +, T = 1) meson exchange, such as are commonly employed in one-boson exchange models for the force, are roughly fitted to the total two-pion exchange potential.

A relativistic and gauge-invariant model of nucleon-nucleon bremsstrahlung

Nucleon-nucleon bremsstrahlung cross sections are calculated in the framework of the one-boson exchange model in a fully relativistic and gauge-in-variant manner. For coplanar and non-coplanar events and also for the Rochester geometry in the proton-proton as well as the proton-neutron case the agreement with the experimental data is excellent for energies above 30 MeV. A strong dependence of the neutron-proton bremsstrahlung cross sections on the polar angle and on the non-coplanarity has been found with interesting implications for future experiments.

Nucleon-hyperon scattering in a multichannel formalism

In the framework of the one-boson exchange model differential and total cross-sections have been calculated for the following processes: ΛN → ΛN, ΛN → ΣN, ΣN → ΣN. Special attention is paid to the coupling of different channels, treated in theND−1 formalism. The parameters are fixed bySU3 symmetry except for the ω and the σ. A reasonable fit with experimental results is obtained.

Λ - p scattering and the Λ - N interaction

On the effective range approximation, an analysis is made of the available Λ-p scattering data using Yukawa Λ - N interactions, including the purely attractive ones as well as those with a hard core. The analysis indicates the appropriateness of a value of the intrinsic range of the Λ-N interaction, approximately that suggested by the one-boson exchange model (boson mass ⪆ 400 MeV). Singlet and triplet Λ-N potentials are also derived and discussed.