Pseudovector Coupling Research Articles

Effects of Perturbative Exchanges in a QCD String Model

The QCD string model for baryons derived by Yu. A. Simonov and used for the calculation of baryon magnetic moments in a previous paper is extended to include also perturbative gluon and meson exchanges. The mass spectrum of the baryon multiplet is studied. For the meson interaction, either pseudoscalar or pseudovector coupling is used. Predictions are compared with the experimental data. Besides these exchanges, the influence of excited quark orbitals on the baryon ground state are considered by performing a multichannel calculation. The nucleon-Δ splitting increases due to the mixing of higher quark states, while the baryon magnetic momenta decrease. The multichannel calculation with perturbative exchanges is shown to yield reasonable magnetic moments, while the mass spectrum is close to experiment.

Pion rescattering inπ0production

This work discusses the approximations frequently used to calculate the contribution to pion production in proton-proton scattering near threshold from irreducible pion re-scattering. The reference result is obtained from the quantum mechanical reduction of the corresponding Feynman diagram to Time-Ordered Field Theory diagrams. We conclude that the Distorted Wave Born Approximation is quite adequate at threshold energies and even above. We discuss the choices for the energy of the exchanged pion, both for its propagator and for the piN amplitude describing the re-scattering vertex. The calculation employs a physical model for nucleons and pions -- pseudo-vector coupling for the piNN vertex and realistic amplitudes for the piN re-scattering and for the NN transitions in the initial and final states.

Gamow-Teller states in relativistic nuclear models

The Gamow-Teller(GT) states are investigated in relativistic models. The Landau-Migdal(LM) parameter is introduced in the Lagrangian as a contact term with the pseudo-vector coupling. In the relativistic model the total GT strength in the nucleon space is quenched by about 12% in nuclear matter and by about 6% in finite nuclei, compared with the one of the Ikeda-Fujii-Fujita sum rule. The quenched amount is taken by nucleon-antinucleon excitations in the time-like region. Because of the quenching, the relativistic model requires a larger value of the LM parameter than non-relativistic models in describing the excitation energy of the GT state. The Pauli blocking terms are not important for the description of the GT states.

RELATIVISTIC DESCRIPTION OF NUCLEAR MATTER IN HARTREE–FOCK APPROXIMATION USING A GENERALIZED HYBRID DERIVATIVE COUPLING

The effect of the exchange terms in nucleon-nucleon interaction is studied for nuclear matter and neutron matter in the framework of the hybrid derivative coupling model. Pseudovector coupling between the pion and the nucleon and both vector and tensor couplings between the rho-meson and the nucleon have been included. A reduced value of the effective nucleon mass is obtained in nuclear matter as a result of the inclusion of the exchange terms. The rho-meson coupling constant obtained from meson exchange theories can be used to get a reasonable value of symmetry energy. The maximum limit of the neutron star mass obtained in this work is more than twice the solar mass. This rather high limit points to the inadequacy of the present Lagrangian at such a high density as it is restricted only to nucleonic degrees of freedom.

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.

The coupling of η mesons to quarks and baryons from [formula omitted] decay

The known ratio of the branching fractions for D s ∗→D sπ 0 and D s ∗→D sγ may be used to extract the coupling of η mesons to strange quarks once the value of the π 0– η mixing angle is known. This requires that realistic models for the spectra as well as the magnetic dipole (M1) decays of the heavy-light ( Q q ̄ ) mesons are available. The coupling of η mesons to light quarks may then be estimated using SU(3) flavor symmetry. Applied to the quark model for the baryons, an ηNN pseudovector coupling constant of f ηNN =0.35 +0.15 −0.25 is obtained. If the charm quark couples significantly to the η meson, as is suggested by the decay mode ψ′→ J/ ψη, then somewhat larger values of f ηNN can be obtained. These values are sufficiently small to be consistent with phenomenological analysis of photoproduction of the η on the nucleon and the reaction pp→ ppη.

Nucleon mass and pion loops

Poincare' covariant Faddeev equations for the nucleon and Delta are solved to illustrate that an internally consistent description in terms of confined-quark and nonpointlike confined-diquark-correlations can be obtained. pi N-loop induced self-energy corrections to the nucleon's mass are analysed and shown to be independent of whether a pseudoscalar or pseudovector coupling is used. Phenomenological constraints suggest that this self-energy correction reduces the nucleon's mass by up to several hundred MeV. That effect does not qualitatively alter the picture, suggested by the Faddeev equation, that baryons are quark-diquark composites. However, neglecting the pi-loops leads to a quantitative overestimate of the nucleon's axial-vector diquark component.

Derivative coupling model description of nuclear matter in the Dirac-Hartree-Fock approximation

Properties of nuclear matter are calculated in the Dirac-Hartree-Fock approximation within the Zim\'anyi and Moszkowski model, which considers a derivative coupling in the scalar field. The present model improves the previous $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\omega}$ version, by the inclusion of the charged mesons $\ensuremath{\pi}$ and $\ensuremath{\rho}.$ The $\ensuremath{\pi}N$ interaction is a mixing of pseudoscalar and pseudovector couplings. The introduction of the $\ensuremath{\pi}$ degrees of freedom has a strong influence on the nucleon effective mass, while keeping the compression modulus in a range of values in agreement with experiments. Effects on the spin-orbit splitting are also discussed.

Pseudovector versus pseudoscalar coupling in kaon photoproduction reexamined

The question of pseudovector versus pseudoscalar coupling schemes for the kaon-hyperon-nucleon interaction is re-examined for the reaction $\gamma p\to K^+ \Lambda$ in several isobaric models. These models typically include Born terms, $K^*$- and $K_1$-exchange in the t-channel, and a few different combinations of spin-1/2 baryon resonances in the $s$- and $u$-channels. The coupling constants are obtained by fitting to a large data set. We find that both pseudoscalar and pseudovector couplings can allow for a satisfactory description of the present database. The resulting coupling constants, $g_{K\Lambda N}$ and $g_{K\Sigma N}$, in the pseudovector coupling scheme are smaller than those predicted using flavor SU(3) symmetry, but consistent with the values obtained in a QCD sum rule calculation.

Isoscalar giant quadrupole resonance state in a relativistic approach with the momentum-dependent self-energies

We study the excited energy of the isoscalar giant quadrupole resonance with the scaling method in the relativistic many-body framework. In this calculation we introduce the momentum-dependent parts of the Dirac self-energies arising from the one-pion exchange on the assumption of the pseudo-vector coupling with nucleon field. It is shown that this momentum-dependence enhances the Landau mass significantly and thus suppresses the quadrupole resonance energy even giving the small Dirac effective mass which causes a problem in the momentum-independent mean-field theory.

Meson-Nucleon Scatterings in an SU(3) × SU(3) σ Model**The project supported by National Natural Science Foundation of China and by the Cao Guang Biao Foundation of Tsinghua University

Meson-nucleon scatterings are investigated in an SU(3) × SU(3) σ model with scalar, vector, pseudoscalar and pseudovector couplings between mesons and baryons. From the calculation of effective coupling constants, sigma terms and s-wave scattering lengths for πN and K+N scatterings in the tree approximation, we find that the pseudoscalar coupling dominates s- and u-channel baryon Born terms, and the cancellation between the scalar and vector interactions leads to reasonable meson-nucleon scattering properties.

Consistent off-shellπNNvertex and nucleon self-energy

We present a consistent calculation of half-off-shell form factors in the pion-nucleon vertex and the nucleon self-energy. Numerical results are presented. Near the on-shell point the pion-nucleon vertex is dominated by the pseudovector coupling, while at large nucleon invariant masses we find a sizable pseudoscalar admixture.

Proton-proton bremsstrahlung calculation: Studies of the off-shell proton electromagnetic vertex and of pseudoscalar vs pseudovectorπNcouplings

The proton-proton bremsstrahlung $(pp\ensuremath{\gamma})$ process has been used as a tool to study the problems related to both the off-shell proton electromagnetic vertex and pseudoscalar (ps)-pseudovector (pv) $\ensuremath{\pi}N$ coupling. We have developed an approach which can be applied to investigate the two problems together. We show that (i) the analyzing powers calculated using the on-shell $p\ensuremath{\gamma}p$ vertex, with ps or pv coupling, yield very poor results for most of the cases with small proton scattering angles at 280 MeV, (ii) the calculations using the off-shell $p\ensuremath{\gamma}p$ vertex and ps coupling in most cases produce cross sections which fail to fit the TRIUMF data at 200 and 280 MeV, and (iii) the data for both cross sections and analyzing powers in the energy region between 157 and 280 MeV can only be consistently described by the calculations using the off-shell $p\ensuremath{\gamma}p$ vertex and pv coupling. Thus our results indicate that the off-shell $p\ensuremath{\gamma}p$ vertex must be used in relativistic $\mathrm{pp}\ensuremath{\gamma}$ calculations and the pseudovector coupling is the best choice for the $\ensuremath{\pi}N$ vertex. More accurate data are needed for further study of the problems.

Electromagnetic Form Factors and Static Properties of the Nucleons in a Chiral Symmetric Quark Model

Nucleon charge and magnetic form factors [Formula: see text] are computed in an independent-quark model with an equally mixed scalar and vector confining potential in square root form taking into account the corrections due to center-of-mass motion and pion-clound effects. The values obtained for the nucleon static properties such as the magnetic moment, charge radius and axial vector coupling constant in the neutron beta decay process agree reasonably with the corresponding experimental data. The pseudo-scalar and pseudo-vector pion-nucleon coupling constants are found to be gNNπ=13.43 and fNNπ=0.282, which are in excellent agreement with the experimental data.

Ferromagnetism of axion domain wall

We show that axion domain wall is ferromagnetic in the universe with nonvanishing baryon number or lepton number. It is caused by protons and electrons bounded to the domain wall with their spins aligned. These bound states arise due to attractive potentials generated through pseudo-vector couplings between the fermions and the axion. Using a model of hadronic axions we predict existence of a primordial magnetic field with strength $10^{-13}$ Gauss at recombination.

Kaon photoproduction near threshold and their coupling constants.

The kaon photoproduction is investigated at the threshold energy. We discuss (i) the difference between the pseudoscalar (PS) and the pseudovector (PV) coupling descriptions on the KNY interaction, (ii) the contributions of t-channel poles of vector and axial vector mesons, and (iii) the contributions of nucleon and hyperon resonances to the ${\mathit{E}}_{0}^{+}$ amplitude. We suggest that total cross sections near threshold can be used to estimate the leading coupling constant, ${\mathit{g}}_{\mathit{KN}}$\ensuremath{\Lambda} and to make clear which description is the better of PS and PV coupling schemes for this reaction from future experiments. \textcopyright{} 1996 The American Physical Society.

Antiproton-proton partial-wave analysis below 925 MeV/c.

A partial-wave analysis of all antiproton-proton scattering data below 925 MeV/c antiproton laboratory momentum is presented. The method used is adapted from the Nijmegen phase-shift analyses of pp and np scattering data. The Nijmegen 1993 antiproton-proton database, consisting of 3646 antiproton-proton scattering data, is presented and discussed. The best fit to this database results in chi^2_min/Ndata = 1.043. The pseudovector coupling constant of the charged pion to nucleons is determined to be (f_c)^2 = 0.0732(11) at the pion pole, where the error is statistical.

Inclusive (p, Delta ++) and (p,p' pi + reactions.

Formulas are derived for cross sections for inclusive (p,${\mathrm{\ensuremath{\Delta}}}^{++}$) and (p,p\ensuremath{'} ${\mathrm{\ensuremath{\pi}}}^{+}$) reactions; these are shown to depend on the pion self-energy in the nucleus, and the production and decay vertices of the ${\mathrm{\ensuremath{\Delta}}}^{++}$. The pion self-energy includes both nucleon excitation (${\mathrm{NN}}^{\ensuremath{-}1}$) as well as delta excitation (\ensuremath{\Delta}${\mathrm{N}}^{\ensuremath{-}1}$) in the nucleus. The pion-nucleon couplings in the transition vertex as well as in the self-energy are written using relativistic pseudovector coupling; results are compared with a nonrelativistic reduction. The distortions of the continuum particles are included in a ``distorted-wave Fermi gas'' approximation. Calculations are made for the excitation spectrum of the nucleus, d\ensuremath{\sigma}/d\ensuremath{\omega}, and the angular distribution in the (p,${\mathrm{\ensuremath{\Delta}}}^{++}$) reaction, for proton beam energies from threshold to 3 GeV. For the (p,p\ensuremath{'} ${\mathrm{\ensuremath{\pi}}}^{+}$) reaction cross sections are calculated for the proton spectrum and the distribution d\ensuremath{\sigma}/d${\mathrm{\ensuremath{\Omega}}}_{\mathrm{\ensuremath{\pi}}}$\ensuremath{\Omega}p\ensuremath{'}${\mathrm{dE}}_{\mathrm{p}\ensuremath{'}}$${\mathrm{dE}}_{\mathrm{\ensuremath{\pi}}}$ at fixed values of the proton and proton angles and a fixed value of ${\mathrm{E}}_{\mathrm{p}\ensuremath{'}}$. The latter essentially gives the excitation energy spectrum of the nucleus. Many interesting features are seen in these spectra. The differences in results using relativistic pion-nucleon coupling and its nonrelativistic reduction are shown to increase with the beam energy. Around 400 MeV, there is little difference in the cross sections but at 1 GeV and beyond the difference becomes large, changing both the magnitude and shape of the distributions. The inclusion of distortions reduces the magnitude of the cross section but leaves the shape unchanged.

Temperature and density effects on the one-pion exchange potential

After considering the vacuum polarization and the radiative correction of the nucleon-nucleon-pion (NN\ensuremath{\pi}) system, the one-pion exchange potential with pseudovector coupling or pseudoscalar coupling at finite temperature and finite density is investigated. By using finite-temperature and density quantum field theory, we find that both the pseudovector and pseudoscalar effective coupling of NN\ensuremath{\pi} decrease as temperature and/or density increase, and drop remarkably at high temperature regions and/or high density regions. At a critical temperature ${\mathit{T}}_{\mathit{c}}$ or a critical density ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$, the effective NN\ensuremath{\pi} coupling diminishes to zero and the one-pion exchange potential vanishes.

The finite-temperature one-pion exchange potential with vertices corrections

The temperature effects of the three-line vertices graph on the one-pion exchange potential (OPEP) with the pseudoscalar and pseudovector couplings is investigated using the method of Matsubara imaginary-time Green function. It is found that the effective coupling of nucleon-nucleon-pion (NN pi ) decreases with temperature and drops rapidly at high temperature. At a critical temperature Tc, the effective NN pi coupling diminishes to zero and the one-pion exchange potential vanishes. Tc is determined to be 215 MeV. OPEP with both mass and vertices corrections are obtained.