Omega Exchange Research Articles

Nucleon-nucleon potential from holography

In the holographic model of QCD, baryons are chiral solitons sourced by D4 flavor instantons in bulk of size 1/λ1/2 with λ = g2Nc. Using the ADHM construction we explicit the exact two-instanton solution in bulk. We use it to construct the core NN potential to order Nc/λ. The core sources meson fields to order (Nc/λ)1/2 which are shown to contribute to the NN interaction to order Nc/λ. In holographic QCD, the NN interaction splits into a small core and a large cloud contribution in line with meson exchange models. The core part of the interaction is repulsive in the central, spin and tensor channels for instantons in the regular gauge. The cloud part of the interaction is dominated by omega exchange in the central channel, by pion exchange in the tensor channel and by axial-vector exchange in the spin and tensor channels. Isovector meson exchanges are subdominant in all channels.

Calculation of the nucleon-nucleon interaction due to vector-meson exchange.

We make use of a momentum-space bosonization of a generalized Nambu--Jona-Lasinio model to calculate the contribution of rho and omega exchange to the one-boson-exchange (OBE) model of the nucleon-nucleon interaction. Momentum-dependent meson-quark coupling constants are obtained in the bosonization scheme. A vector-meson-dominance (VMD) model is used to obtain information concerning the momentum dependence of the meson-nucleon vertex, other than that which arises from the momentum dependence of the meson-quark coupling constants. We find good agreement with the magnitude of the force at ${\mathit{q}}^{2}$=0 for both rho and omega exchange. The momentum dependence of the interaction in the region -0.2 ${\mathrm{GeV}}^{2}$\ensuremath{\le}${\mathit{q}}^{2}$\ensuremath{\le}0 was calculated. We only obtain about two-thirds of the strength of the OBE interaction at ${\mathit{q}}^{2}$=-0.2 ${\mathrm{GeV}}^{2}$, suggesting the importance of interactions of shorter range than that considered here. (We note that, for -0.2 ${\mathrm{GeV}}^{2}$\ensuremath{\le}${\mathit{q}}^{2}$\ensuremath{\le}0, we span the range of ${\mathit{q}}^{2}$ of significance for nuclear structure studies.) \textcopyright{} 1996 The American Physical Society.

Bosonization in the presence of confinement: Calculation of the nucleon-nucleon interaction.

We describe an extended version of the Nambu\char21{}Jona-Lasinio (NJL) model that includes a description of confinement. It is necessary to incorporate some description of confinement in order to discuss the properties of the sigma, rho, and omega mesons in the NJL model. These mesons, in addition to the pion, are the minimum needed to describe the salient features of the nucleon-nucleon interaction. In previous work we considered the relation between the bosonized NJL model and the one-boson-exchange (OBE) model of the nucleon-nucleon force. Most of our attention was given to pion and sigma exchange. We provide a review of that work and extend our discussion to a consideration of rho and omega exchange. We also present a more detailed discussion of the bosonization procedure. Our results depend upon the strength of the confining interaction. Once that is fixed, we obtain good values for the omega-nucleon coupling constant, ${\mathit{G}}_{\mathrm{\ensuremath{\omega}}\mathit{NN}}$, and for the tensor coupling constant ${\mathit{f}}_{\mathrm{\ensuremath{\rho}}}$, in the rho-nucleon interaction. (One limitation of the present version of the model is that the ratio ${\mathit{f}}_{\mathrm{\ensuremath{\rho}}}$/${\mathit{g}}_{\mathrm{\ensuremath{\rho}}}$=3.70, instead of the empirical value of ${\mathit{f}}_{\mathrm{\ensuremath{\rho}}}$/${\mathit{g}}_{\mathrm{\ensuremath{\rho}}}$\ensuremath{\simeq}6.1.) If we consider nucleon-nucleon scattering for relatively small momentum transfer, we obtain good results for the processes of sigma, pion, rho, and omega exchange. Remarkably, the description of pion exchange is very accurate up to ${\mathit{q}}^{2}$\ensuremath{\sim}-2 ${\mathrm{GeV}}^{2}$. That is, the microscopic model reproduces the pion-exchange amplitude of the boson-exchange model over a broad range of momentum transfer when we specify a single parameter than governs the momentum-transfer dependence of the pseudoscalar-isovector form factor of the nucleon. In the other channels (\ensuremath{\sigma},\ensuremath{\rho},\ensuremath{\omega}), the nucleon form factors may be treated in the same manner. However, if we calculate the form factors in our model, we find that they are too ``soft'' to fit the OBE amplitudes away from ${\mathit{q}}^{2}$\ensuremath{\simeq}0. Further work is needed to obtain good fits for the various amplitudes for large momentum transfer, although the OBE amplitudes are well reproduced in the case of scattering at small momentum transfer (\ensuremath{\Vert}${\mathit{q}}^{2}$\ensuremath{\Vert}\ensuremath{\le}0.1 ${\mathrm{GeV}}^{2}$). \textcopyright{} 1996 The American Physical Society.

Parity-violating longitudinal response

The longitudinal quasielastic parity-violating electron scattering response is explored within the context of a model that builds antisymmetrized RPA-HF correlations on a relativistic Fermi gas basis. The large sensitivity to nuclear dynamics of this observable, found in previous studies where only pionic correlations were included, is shown to survive in the present model where the effects from pion, rho, sigma and omega exchange in a version of the Bonn potential are incorporated. Through an intricate diagrammatic cancellation/filtration mechanism the longitudinal parity-violating response turns out to be close to the one obtained in first-order perturbation theory with the pion alone. Finally, in accord with our previous work, the parity-violating response is seen to display appreciable sensitivity to the electric strangeness content of the nucleon, especially at high momentum transfer.

Masses of hadrons and nucleon-nucleon interaction at finite temperature and density in a quantum hadrodynamics model.

The effective masses and the screening masses of hadrons at finite temperature and density for the quantum hadrodynamics two (QHD-II) model have been investigated under the one-loop approximation. The nucleon-nucleon interaction through pion, sigma, omega, and rho mesons exchange in hot nuclear matter is given. We have found that the potential well of the nucleon-nucleon interaction becomes shallow as the temperature and/or density increases. At a critical temperature and/or a critical density, the potential well disappears.

Effective interaction for relativistic mean-field theories of nuclear structure.

We construct an effective interaction, which when treated in a relativistic Hartree-Fock approximation, reproduces rather accurately the nucleon self-energy in nuclear matter and the Migdal parameters obtained via relativistic Brueckner-Hartree-Fock calculations. This effective interaction is constructed by adding Born terms, describing the exchange of pseudoparticles, to the Born terms of the Dirac-Hartree-Fock analysis. The pseudoparticles have relatively large masses and either real or imaginary coupling constants. (For example, exchange of a pseudo-sigma with an imaginary coupling constant has the effect of reducing the scalar attraction arising from sigma exchange while exchange of a pseudo-omega with an imaginary coupling constant has the effect of reducing the repulsion arising from omega exchange. The terms beyond the Born term in the case of pion exchange are well simulated by pseudo-sigma exchange with a real coupling constant.) The effective interaction constructed here may be used for calculations of the properties of finite nuclei in a relativistic Hartree-Fock approximation.

Nucleon-nucleon force in a skyrmion model stabilized by omega exchange.

We consider a model of nucleon-nucleon interactions in which the nucleon is a Skyrme soliton stabilized by \ensuremath{\omega}-meson exchange, rather than the usual quartic Lagrangian term. Making the product approximation for the meson field matrix U and an additive approximation for the \ensuremath{\omega} field, we obtain nonzero, central, spin-spin, and tensor NN potentials. Their magnitudes, but not shapes, are sensitive to the \ensuremath{\omega} coupling constant. In general, the results are similar to others obtained, using the same kinds of approximations, with the usual Skyrme Lagrangian. Neither approach finds intermediate-range attraction in the I=0 central potential.

Proton-proton potential models obtained from the Virginia phase-shift analysis

The results of a recent phase-shift analysis of the experimental data have been used to construct equivalent local potential models for the low angular momentum states of the proton-proton system. The equivalent local potential for the 1S0 state is found to be quite different from that obtained from the Livermore phase-shifts and to be an improvement on the Reid 1S0 potential at intermediate distances. The difficulties experienced with the previous 3P0 and 3P1 phase-shifts are not encountered with the new data. The triplet tensor potential differs from most other phenomenological results but is similar to that of the Paris potential and thus reflects the effects of omega exchange. A simple nonlocal potential for the set of even states 1S0, 1D2 and 1G4 and one for the set of odd states 3P0, 3P1, 3P2-3F2, 3F3 and 3F4-3H4 are presented.

Coherent K ∗ (890) production off deuterium

We have studied 76 events of the reaction K − d → d K ̄ 0π − at 4.5 GeV/ c. The reaction is dominated by the production of K ∗ (890) and is analyzed in terms of omega exchange. The density matrix elements are obtained.

Shrinkage of the Diffraction Pattern and Short-Range Forces

ABS>The observed shrinkage of the diffraction pattern in p-p, but not in pi /sup -/--p, scattering is considered. Representation of the pi /sup -/-p differential cross section in terms of the Pomeranchuk trajectory alone is shown to give good agreement with diffraction data. In p-p scattering the Pomeranchuk term determines the low momentum transfer behavior of the cross section, while at about 10 Bev/c the term due to omega exchange (not present in pi /sup -/-p) becomes important. omega -exchange is also present in K/sup plus or minus /-p scattering; this diffraction pattern should show shrinkage similar to the p-p and p-p shrinkage. (D.C.W.)