One-boson Exchange Research Articles

Interactions of the solar neutrinos with the deuterons

Starting from chiral Lagrangians, possessing the SU(2)_L x SU(2)_R local chiral symmetry, we derive weak axial one-boson exchange currents in the leading order in the 1/M expansion (M is the nucleon mass). We apply these currents in calculations of the cross sections for the disintegration of the deuterons by the low energy neutrinos. The nuclear wave functions are derived from a variant of the OBEPQB potential and from the Nijmegen 93 and Nijmegen I nucleon-nucleon interactions. The comparison of our cross sections with those obtained within the pionless effective field theory and other potential model calculations shows that the solar neutrino-deuteron cross sections can be calculated within an accuracy of 3.3 %.

Nucleon-sigmacoupling constant in QCD sum rules

The external-field QCD sum rules method is used to evaluate the coupling constant of the light isoscalar-scalar meson (``\ensuremath{\sigma}'' or \ensuremath{\varepsilon}) to the nucleon. The contributions that come from the excited nucleon states and the response of the continuum threshold to the external field are calculated. The obtained value of the coupling constant is compatible with the large value required in one-boson exchange potential models of the two-nucleon interaction.

Nonlocality in the nucleon-nucleon interaction and nuclear matter saturation

We study the possible relationship between the saturation properties of nuclear matter and the inclusion of non-locality in the nucleon-nucleon interaction. To this purpose we compute the saturation curve of nuclear matter within the Bethe-Brueckner-Goldstone theory using a recently proposed realistic non-local potential, and compare it with the corresponding curves obtained with a purely local realistic interaction (Argonne v$_{18}$) and the most recent version of the one-boson exchange potential (CD Bonn). We find that the inclusion of non-locality in the two-nucleon bare interaction strongly affects saturation, but it is unable to provide a consistent description of few-body nuclear systems and nuclear matter.

Relativistic Mean Field Models and Nucleon-Nucleon Interactions

We investigate the connection between relativistic mean field models and the bare nucleon-nucleon interaction by using a realistic interaction in the nuclear medium. Starting from a nonrelativistic bare potential and by employing a G-matrix formalism we derive an effective interaction in the nuclear medium that depends on its density. We show that its medium- and long-range components can be described to a good extent by an effective density-dependent one-boson-exchange (OBE) potential. For a good fit of the OBE to the G-matrix potential the following degrees of freedom have been found to be necessary: an isoscalar scalar S (σ-meson), an isoscalar vector V (ω-meson), an isovector scalar TS (δmeson), an isovector vector TV (ρ-meson), and an isovector pseudoscalar PS (π-meson) Yukawa field. This allows us to extract the mass and the coupling strength of the various mesons at different densities of nuclear matter. In such a way, we investigate the densitydependence of these parameters and the influence of many-body and multi-scattering effects. We find that with increasing density of the nuclear medium mσ is constant to a good approximation, mδ decreases quite strongly, while mω and mρ decrease rather weakly. The coupling strengths of the fields also show a rather weak decreasing tendency. Furthermore it turns out that the extracted values of the meson fields masses and couplings are rather consistent with the parameters of effective mesons of recent RMF phenomenologies. Finally, we find that the tensor force of the bare nucleon-nucleon interaction is essential to explain the dominance of the σ-field for the attraction. This makes RMF models more consistent with ab initio calculations.

The Mainz A4 parity violation experiment (single spin asymmetries in elastic scattering)

The A4 experiment at the MAMI facility in Mainz measures a parity violating single spin asymmetry in the scattering of longitudinally polarized electrons off unpolarized protons. The longitudinal single spin asymmetry is parity violating and arises from the interference of the weak and electromagnetic one-boson exchange amplitude. It is sensitive to the strangeness contribution to the electromagnetic form factors of the nucleon. Recently the transverse single spin asymmetry has gained much interest. It is purely electromagnetic in origin and arises from the interference of the one-photon with the two-photon exchange amplitude. It is a time-odd quantity and sensitive to the imaginary part of the two-photon amplitude only. The A4 collaboration has measured both asymmetries at two different momentum transfer values. The physical background, the experimental apparatus, the experimental results and the implications are discussed.

Soft-photon analysis of nucleon-nucleon bremsstrahlung: Anomalous magnetic moment effects

Two soft-photon amplitudes, the Low amplitude and the two-$u$--two-$t$ special amplitude, agree with one another in the case of $np\ensuremath{\gamma}$ but disagree significantly in the case of $pp\ensuremath{\gamma}$. The two-$u$--two-$t$ special amplitude describes well the available $pp\ensuremath{\gamma}$ cross section data. The relationship between the two amplitudes as well as the reason they agree and disagree is explored. Using these two soft-photon amplitudes plus a one-boson exchange amplitude, the contribution of the proton and neutron anomalous magnetic moments to $pp\ensuremath{\gamma}$ and $np\ensuremath{\gamma}$ has been investigated for projectile energies above $150\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and for laboratory scattering angles lying between $8\ifmmode^\circ\else\textdegree\fi{}$ and $35\ifmmode^\circ\else\textdegree\fi{}$. The anomalous magnetic moment contribution was found to be significant in the $pp\ensuremath{\gamma}$ process but insignificant in the $np\ensuremath{\gamma}$ process. Additional aspects of the bremsstrahlung mechanism are discussed. Our findings play an important role in understanding the similarities and differences between the Low amplitude and the two-$u$--two-$t$ special amplitude as well as why the two-$u$--two-$t$ special amplitude calculation agrees well with the available $pp\ensuremath{\gamma}$ cross section data.

Phase Transition of Hot Nuclear Matter

Based on the difference between the Fermi distributionsat zero temperature and at the finite temperature, we introduce thetemperature-dependent three-body force (TBF) into the microscopicfinite-temperature Brueckner–Hartree–Fock theory (FTBHF). In terms ofthe meson-exchange current approach, i.e. the one boson exchange (OBE)approximation, the exchange of four important mesons π, ρ,σ and ω are considered. Using the FTBHF theoryincluding TBF, we describe the critical temperature of the liquid–gasphase transition for symmetric nuclear matter and discuss its changetrend with the increasing asymmetry parameter. Compared to the resultexcluding TBF, the value of the critical temperature turns out to besmaller.

Quark-Model Baryon-Baryon Interaction and Its Applications to Hypernuclei

The quark-model baryon-baryon interaction fss2, proposed by the Kyoto-Niigata group, is a unified model for the complete baryon octet (B_8=N, Lambda, Sigma and Xi), which is formulated in a framework of the (3q)-(3q) resonating-group method (RGM) using the spin-flavor SU_6 quark-model wave functions and effective meson-exchange potentials at the quark level. Model parameters are determined to reproduce properties of the nucleon-nucleon system and the low-energy cross section data for the hyperon-nucleon scattering. Due to the several improvements including the introduction of vector-meson exchange potentials, fss2 has achieved very accurate description of the NN and YN interactions, comparable to various one-boson exchange potentials. We review the essential features of fss2 and our previous model FSS, and their predictions to few-body systems in confrontation with the available experimental data. Some characteristic features of the B_8 B_8 interactions with the higher strangeness, S=-2, -3, -4, predicted by fss2 are discussed. These quark-model interactions are now applied to realistic calculations of few-body systems in a new three-cluster Faddeev formalism which uses two-cluster RGM kernels. As for the few-body systems, we discuss the three-nucleon bound states, the Lambda NN-Sigma NN system for the hypertriton, the alpha alpha Lambda system for 9Be Lambda, and the Lambda Lambda alpha system for 6He Lambda Lambda.

Two-fermion relativistic bound states in light-front dynamics

In the Light-Front Dynamics, the wave function equations and their numerical solutions, for two fermion bound systems, are presented. Analytical expressions for the ladder one-boson exchange interaction kernels corresponding to scalar, pseudoscalar, pseudovector and vector exchanges are given. Different couplings are analyzed separately and each of them is found to exhibit special features. The results are compared with the non relativistic solutions.

Sensitivity ofppbremsstrahlung on low-energyNNinteraction

In this paper we investigate possible reasons for the differences found in some kinematic regions between existing microscopic pp bremsstrahlung models and experimental data. It is shown that this is partly the result of an inaccurate description of the elastic nucleon-nucleon (NN) T matrix at low energies. We show that for the phase space probed by the recent KVI experiment, Coulomb corrections do not influence the observables. The difference between theory and experiment is reduced after the NN one-boson exchange model is refitted to the pp phase shifts, however, a sizable discrepancy persists.

Nuclear spin-isospin correlations, parity violation, and thefπproblem

The strong interaction effects of isospin- and spin-dependent nucleon-nucleon correlations observed in many-body calculations are interpreted in terms of a one-pion exchange mechanism. Including such effects in computations of nuclear parity violating effects leads to enhancements of about 10%. A larger effect arises from the one-boson exchange nature of the parity non-conserving nucleon- nucleon interaction, which depends on both weak and strong meson-nucleon coupling constants. Using values of the latter that are constrained by nucleon-nucleon phase shifts leads to enhancements of parity violation by factors close to two. Thus much of previously noticed discrepancies between weak coupling constants extracted from different experiments can be removed.

Λ-Λ interaction andΛΛ6He

A one-boson exchange (OBE) potential model, based on the Nijmegen model D potential, for the ${}^{1}{S}_{0},$ $S=\ensuremath{-}2$ interaction is analyzed with emphasis on the role of coupling between the \ensuremath{\Lambda}\ensuremath{\Lambda}, $N\ensuremath{\Xi},$ and \ensuremath{\Sigma}\ensuremath{\Sigma} channels. Singlet scalar exchange, an approximation to two-pion exchange, is significant in all channels; surprisingly, the one-pion exchange component is almost negligible. The size of the channel coupling as a function of the overall strength of the OBE model potential is examined. Implications of the analysis for the binding energy of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}\mathrm{He}$ are considered; the new experimental datum may suggest a consistency between the extracted \ensuremath{\Lambda}\ensuremath{\Lambda} matrix element and the relation implied by SU(3) among OBE baryon-baryon interactions.

Pseudovector vs pseudoscalar coupling in one-boson exchangeNNpotentials

We examine the effects of pseudoscalar and pseudovector coupling of the pi and eta mesons in one-boson exchange models of the NN interaction using two approaches: time-ordered perturbation theory unitarized with the relativistic Lippmann-Schwinger equation, and a reduced Bethe-Salpeter equation approach using the Thompson equation. Contact terms in the one-boson exchange amplitudes in time-ordered perturbation theory lead naturally to the introduction of s-channel nucleonic cutoffs for the interaction, which strongly suppresses the far off-shell behavior of the amplitudes in both approaches. Differences between the resulting NN predictions of the various models are found to be small, and particularly so when coupling constants of the other mesons are readjusted within reasonable limits.

What does the free space Lambda Lambda interaction predict for Lambda Lambda hypernuclei?

Data on LambdaLambda hypernuclei provide a unique method to learn details about the strangeness S = -2 sector of the baryon-baryon interaction. From the free space Bonn-Jülich potentials, determined from data on baryon-baryon scattering in the S = 0,-1 channels, we construct an interaction in the S = -2 sector to describe the experimentally known LambdaLambda hypernuclei. After including short-range (Jastrow) and RPA correlations, we find masses for these LambdaLambda hypernuclei in a reasonable agreement with data, taking into account theoretical and experimental uncertainties. Thus, we provide a natural extension, at low energies, of the Bonn-Jülich one-boson exchange potentials to the S = -2 channel.

Relativistic field-theoretical representation of the Schrödinger equation and field-theoretical generalization of the inverse scattering method

The relativistic three-dimensional equation ( 2 E p ′ − 2 E p ) 〈 p ′ / Ψ p 〉 = − ∫ V ( t ) d 3 p ″ 〈 p ″ / Ψ p 〉 with E p = m 2 + p 2 and t = ( E p′ − E p″ ) 2−( p′− p″) 2 is transformed into Schrödinger equation (Δ r + k 2)〈 r|ϕ k 〉=− v( r)〈 r|ϕ k 〉 with E p = k 2/2 m+ m using the Fourier transformation. The potential v( r) and the wave function 〈 r|ϕ k 〉 differ from the corresponding Fourier images of the relativistic potential V( t) and the relativistic wave function 〈 p′| Ψ p 〉 with the kinematical factors only. In the framework of the standard field-theoretical approach the three-dimensional relativistic equations for the NN scattering amplitude with the total NN potential V tot = V( t)+ V nonl are derived. The nonlocal part of this NN potential V nonl is constructed from the πNN vertex functions which are determined from the πN phase shifts. The other part of the NN potential V( t) consists of the π-, σ-, ρ-, ω-, … meson exchange diagrams and for the NN contact (overlapping) terms. In the case of the renormalizable Lagrangians V( t) reproduces exactly the One Boson Exchange (OBE) Bonn model of the NN potential. The inverse scattering problem for this V( t) potential is reduced to the construction of the potential v( r) of the ordinary Schrödinger equation from the NN phase shifts.

Effects of negative-energy components in two-body deuteron photodisintegration

Several observables in two-body deuteron photodisintegration are investigated in the framework of the Bethe-Salpeter formalism. Apart from keeping the Lorentz covariance throughout, special attention is paid to inclusion of both the positive-energy and negative-energy partial-wave components of the deuteron state. Using the Bethe-Salpeter equation for the deuteron in the ladder approximation with one-boson exchanges as a driving term, the contribution of the negative-energy states is studied for the unpolarized differential cross section as well as for linear photon and tensor target asymmetries. These states are found to have an impact on the observables and, thus, should be taken into account in a complete theoretical treatment of the reaction in the intermediate energy regime.

Comparison between chiral and meson-theoretic nucleon-nucleon potentials through(p,p′)reactions

We use proton-nucleus reaction data at intermediate energies to test the emerging new generation of chiral nucleon-nucleon (NN) potentials. Predictions from a high quality one-boson-exchange (OBE) force are used for comparison and evaluation. Both the chiral and OBE models fit NN phase shifts accurately, and the differences between the two forces for proton-induced reactions are small. A comparison to a chiral model with a less accurate NN description sets the scale for the ability of such models to work for nuclear reactions.

The Effective Degree of Freedom of Low Energy QCD**The project partly supported by NSF, SED and SSTD of China, and partly by the US Department of Energy under contract W-7405-ENG-36

The adiabatic effective baryon-baryon interactions and dibaryon candidates are studied systematically with three constituent quark models based on different effective degrees of freedom: Glozman–Riska–Brown Goldstone boson exchange model based on constituent quark and Goldstone boson coupling; Fujiwara model based on constituent quark gluon coupling and Nijmegen one-boson exchange; QDCSM based on constituent quark and gluon coupling with quark delocalization and color screening. We find that the three models predicted the similar effective baryon-baryon interactions for roughly two thirds among the 64 states consisted of octet and decuplet baryons. The differences among three models and their separate characteristics are also studied.

FourthP11resonance predicted by the constituent quark model

We point out a distinguishing difference between constituent quark models based on one-gluon exchange and one-boson exchange dynamics. In the latter one, the P_11 nucleon resonance with predominantly symmetrical spatial wave function in the N=4 band gets strongly attracted such that it drops below some states in the N=2 band. Calculations of strong decay widths are presented in order to establish an identification with experimental states. Our results are relevant for the interpretation of the fourth P_11 resonance that was found in the partial wave analysis of the Zagreb group and recently discussed by Capstick et. al. in the framework of a model based on one-gluon exchange.

NUCLEON-NUCLEON AND HYPERON-NUCLEON INTERACTIONS

The nucleon-nucleon and hyperon-nucleon interactions are studied by the one-boson-exchange(OBE) potential with the vertex form factor. To make our theory as relativistic as possible, we calculate the OBE-potential in the momentum space and use the Blankenbecler-Sugar equation. We also treat the singularity in the multichannel Blankenbecler-Sugar equation. By these formulations, we analyze the nucleon-nucleon and the hyperon-nucleon scatterings.