The meson-exchange magnetic moment operator in heavy nuclei

The form of the isovector meson-exchange magnetic moment operator for heavy nuclei is derived in a simple isobar-hole model for the meson-induced polarization of the nuclear medium. It is shown that the model-independent parts of the pion and ρ-meson exchange-current contributions are rather little changed by the medium polarization mechanisms whereas the model-dependent parts that involvegD³³ intermediate states are strongly quenched. Because of cancellations between the different exchange mechanisms the net meson-exchange contribution is appreciably changed by the medium polarization. An effective one-body operator that describes the meson-exchange contribution is constructed for closed-shell + one-particle nuclei.     

Pionic disintegration of the deuteron

Pionic disintegration of the deuteron between threshold and the 3,3 resonance region is described by a model containing one- and two-body absorption. The two-body absorption mechanism is due to pion and ?-meson rescattering calculated from phenomenological Lagrangians. The role of the ?-meson is crucial in reducing the cross section due to pion exchange. The role of the mass distribution of the ρ-meson, hadronic form factors and final state interactions are investigated. Good agreement with empirical results for the total cross section is obtained with a set of currently accepted values for the meson-nucleon coupling constants and the ρ-mass distribution.     

Ring diagrams,Landau parameters,pion condensation and the binding energy of nuclear matter

We calculate all direct π-exchange ring diagrams to arbitrary orders in nuclear matter. Our model incorporates intermediate Δ(1236) resonances, correlations, ρ- as well as π-meson exchange and mesonic form factors. We find that the convergence of the diagram summation is governed by two quantities which have an immediate physical interpretation: the Landau parameter G'₀ and the threshold for pion condensation. The contribution to the energy of nuclear matter from terms of third and higher order is 2 MeV/particle (repulsion) at normal density, with a strong density dependence.     

Anomalous quark-gluon chromomagnetic interaction and high-energy ρ-meson electroproduction

It is shown that existence of a large anomalous chromomagnetic moment of quark induced by nonperturbative structure of QCD leads to the additional contribution to exclusive ρ-meson electroproduction off proton target. The contribution coming from new type of quark-gluon interaction to the ρ-meson production cross section for both transversal and longitudinal polarization of virtual photon is found. Such nonperturbative contribution together with conventional perturbative two-gluon exchange allows us to describe the experimental data at low Q ² for transversal polarization. However, in the longitudinal polarization case there is still some discrepancy with the data. The possible source of this deviation is discussed.     

The magnetic form factor of He

The behavior of the magnetic form factor of ³He for momentum transfers q² < 30 fm⁻² is studied with simple wave function models and with account of pion and ρ-meson exchange current effects. The single-nucleon current contribution to the form factor depends strongly on the D-state probability. The exchange current contributions dominate the form factor for q² > 5 fm⁻². The sensitivity of the exchange current contributions to wave function details — short-range correlations and D-state percentage — and hadronic form factors in the exchange operators is investigated in some detail. With inclusion of the ρ-meson exchange effect the predicted position of the form factor minimum is in reasonable agreement with the most recent data. With the exchange effects considered the predicted magnetic moment agrees with the empirical value. The models employed lead to no noticeable difference in the behavior of the triton and helion magnetic form factors.     

The Lorentz-Lorenz correction

The Ericson-Ericson Lorentz-Lorenz correction in the propagation of pions in matter is derived in a relativistic theory for general pion frequency and wave vector, in terms of the nucleon-nucleon pair correlation function. The relation to the electromagnetic Lorentz-Lorenz correction is described. It is shown that ρ-meson exchange between nucleons can be expected to give contributions to the Lorentz-Lorenz correction comparable to those from π-mesons. The precise magnitude of the ρ-contributions depends sensitively on the detailed behaviour of the two-body correlation function, since the range of the interaction through ρ-exchange is comparable to that of the repulsive correlations.     

On mesonic interference phenomena in deep inelastic scattering

Possible effects of a π′ contribution to lepton deep-inelastic scattering (DIS) are analyzed. The exchange of the first radial excitation of the pion (π′(0^?)) has been advocated recently to solve the notorious problem of hard vertex form factors innucleon-nucleon scattering. The π′-meson exchange contribution is calculated in addition to the well known pionic (Sullivan) contribution. Interference between π and π′ contributions and their possible influence on the cut-off mass parameter of the πNN form factor extracted from DIS on the nucleon are examined. It is shown that the orthogonality relation between the π and π′ wave functions, in conjunction with number and momentum sum rules, put strong restrictions on the size of the interference effect. The interference effect between π and π′ is found to be negligible for the nucleon quark distributions and to modify only higher moments of the quark distributions. The interference effects of a pionic contribution with other mesons are discussed.     

The role of the form factor and short-range correlation in the relativistic Hartree-Fock model for nuclear matter

The role of the form factor and short-range correlation in nuclear matter is studied within the relativistic Hartree-Fock approximation. We take, first, the mean-field approximation for meson fields and obtain the fluctuation terms of mesons to be used for the Fock energies. We introduce form factors in the meson-nucleon coupling vertices to take into account the finite-size effect of the nucleon. We use further the unitary correlation operator method for the treatment of the short-range correlation. The form factors of the size ( L \Lambda ∼ 1.0 -2.0GeV) of the nucleon-nucleon interaction cut down largely the contribution of the r \rho -meson in the Fock term. The short-range correlation effect is not large but has a significant effect on the pion and r \rho -meson energies in the relativistic Hartree-Fock approximation for nuclear matter.     

Radiative capture of negative pions in deuterium

The transition rate for radiative capture of negative pions in deuterium is calculated with the Hamiltonian deduced from pion photoproduction data and with the transition amplitude obtained on the basis of the soft-pion theory. A large contribution to the physical amplitude from the intermediate states with the ρ-meson and nucleon-antinucleon pairs in the soft-pion approach brings both results into agreement within 5 %.     

A note on scalar meson dominance

We consider chiral perturbation theory with an explicit broad σ-meson and study its contribution to the scalar form factors of the pion and the nucleon. Our goal is to learn more about resonance saturation in the scalar sector.     

Meson-exchange currents in pion-nucleus double-charge-exchange reactions

The scattering of pions by virtual mesons in the nuclear medium is analysed and its contribution to the process of double-charge exchange in pion-nucleus scattering compared to the conventional mechanism which involves charge exchange in the scattering of the pions with two different nucleons. It is shown that at forward angles the cross section is decreased by about 50% around pion energies T ≈ 130 MeV and increased by a similar amount at energies above the resonance, T ≈ 250 MeV. At large angles the meson-exchange contribution changes the cross section appreciably and can become the dominant mechanism.     

One ρ-Meson Exchange Potential at Finite Temperature

By means of the imaginary-time Green function method in finite-temperature field theory,one ρ-meson exchange potential at finite temperature is obtained.It is found that ρ-meson become lighter in terms of temperature increasing.The result is in good agreement with that obtained from scalling property of QCD.It is also found that temperature dependence of ρ-meson coupling constants is similar to that of,σ-meson coupling constants.     

Electron scattering and charged pion photoproduction on12C and13C

Electron scattering and charged pion photoproduction at 200 MeV on¹²C and¹³C have been studied by the nuclear shell model to investigateM1 andE2 nuclear form factors of the isovector type. Starting with the Op-shell model of Hauge-Maripuu, the effects of core polarization by theM 3Y force and the one-pion-exchange current are taken into account. The effect of core polarization is found to be essential in both electron scattering and pion photoproduction, while the meson-exchange current, which also improves the calculated results, gives contributions of about 10%–15%.     

A relativistic description of πd scattering: Effects of small πN waves,NN rescattering, ρ-exchange and π absorption and emission

Using a relativistic three-body theory we calculated elastic differential cross sections, polarisations and total cross sections for pion-deuteron scattering in the (3, 3) resonance region. Effects of pion absorption and emission have been calculated rigorously, including pion rescattering effects and ρ-meson exchange to all orders. Inclusion of pion absorption and emission produces appreciable changes in the large angle differential cross section and in all polarisation parameters. The inclusion of non-resonant πN partial waves leads to significantly improved agreement with experiment for T_π = 142 MeV and at forward angles for 256 MeV. None of these effects, nor the inclusion of selected NN partial waves other than ³S₁-³D₁, produces the deep minimum observed at 256 MeV.     

REPULSIVE CORE STRENGTH OF THE NUCLEON-NUCLEON INTERACTION

The contributions to the repulsive corestrength of the N-N force from both the quarkpion and quark-gluon interactions in the Chiral(or Cloudy) Bag Model are calculated. These contributions are about several hundred MeV,a great part of which comes from the quark-pion interactions. It is found that when self-energies due to both gluon and pion exchange are considered,the contribution from the self-energy due to pion exchang is more important than that from the direct and exchange energy of guarkpion interactions.     

Near-threshold η production in pp collisions

We study near-threshold η meson production in pp collisions within an effective Lagrangian approach combined with the isobar model, by allowing for the various intermediate nucleon resonances due to the π, η, and ρ-meson exchanges. It is shown that the ρ-meson exchange is the dominant excitation mechanism for these resonances,and the contribution from the N*(1720) is dominant. The total cross section data can be reasonably reproduced,and the anisotropic angular distributions of the emitted η meson are consistent with experimental measurements.Besides, the invariant mass spectra of pp and pη explain the data well at excess energy of 15 Me V, and are basically consistent with the data at excess energy of 40 Me V. However, our model calculations cannot reasonably account for the two-peak structure in the pη distribution at excess energies of 57 and 72 Me V, which suggests that a more complicated mechanism is needed at higher energy region.     

Relativistic shell models and meson-exchange currents

We discuss the connection between nonrelativistic pair exchange currents and relativistic mean field theories. Heavy mesons, especially the σ-meson, are shown to yield strong enhancements to the matrix element of “odd” Dirac operators, such as γ and γ₄γ₅, in complete analogy to the strong enhancements obtained in relativistic mean field theories. For isoscalar operators, there is a cancelling contribution from ω-pair diagrams, called “backflow” in relativistic theories, but for isovector operators the equivalent ρ-pair diagram is very weak. Agreement between traditional nonrelativistic calculations and experiment is worsened by the introduction of heavy-meson pair diagrams. The situation can be improved on adding vertex form factors and short-range correlation functions that reduce the heavy-meson pair contribution by about a factor of two. The differences between the two approaches are emphasized and experimental tests are discussed.     

Nucleon polarization and the nuclear charge operator

Effects of nucleon polarization on the nuclear charge operator have been evaluated in a constituent quark model. At momentum transfer q ≈ 4 fm^?1 monopole, dipole and quadrupole excitations are of equal importance. In a harmonic oscillator model for ³He all multipolarities give negative contributions, leading to an overall contribution comparable to the relativistic pair effect. The influence of realistic wave functions, coupling constants and off-shell form factors is discussed.