Improved calculation of some meson exchange current contributions to the magnetic form factor of medium-heavy nuclei

The contribution of some meson exchange currents to the magnetic form factor of ⁴⁹Ti, ⁵¹V, ⁸⁷Sr and ⁹³Nb has been claimed to be important. We have reexamined those claims and conclude that previous models would not explain the experimentally large form factors near q = 3.0 fm^?1. Inclusion of short range NN correlations enhances these meson exchange current contribution by factors of 2.5 to 4.0, but further inclusion of meson nucleon vertex form factors goes in the wrong direction.     

Correlations and the Dirac structure of the nucleon self-energy

The Dirac structure of the nucleon self-energy in symmetric nuclear matter as well as neutron matter is derived from a realistic meson exchange model for the nucleon-nucleon (NN) interaction. It is demonstrated that the effects of correlations on the effective NN interaction in the nuclear medium can be parameterized by means of an effective meson exchange. This analysis leads to a very intuitive interpretation of correlation effects and also provides an efficient parametrization of an effective interaction to be used in relativistic structure calculations for finite nuclei. Received: 29 January 2001 / Accepted: 5 May 2001     

The study of the binding energy of nuclear matter in the relativistic σ− ω− π model with Δ isobar degree of freedom

The relativistic σ?ω?π model is proposed and used to calculate the binding energy of relativistic nuclear matter. By coupling Δ isobar to the σ meson, the zero-point fluctuation energy of the Δ isobar in the one loop approximation is derived. We calculate the effective mass of nucleon and Δ isobar, exchange and correlation energies, pressure and incompressibility of nuclear matter. The density dependence correction to σNN ωNN coupling constants is a very important mechanism to saturate the binding energy. The pion propagator is nuclear matter is constructed by the relativistic particle-hole, delta-hole and short-range correlation. The pion dispersion relation is calculated we find it’s very sensitive to the effective mass of nucleon and Δ isobar.     

General survey of polarization observables in deuteron electrodisintegration

Polarization observables in inclusive and exclusive electrodisintegration of the deuteron using a polarized beam and an oriented target are systematically surveyed using the standard nonrelativistic framework of nuclear theory but with leading-order relativistic contributions included. The structure functions and the asymmetries corresponding to the various nucleon polarization components are studied in a variety of kinematic regions with respect to their sensitivity to realistic NN-potential models, to subnuclear degrees of freedom in terms of meson exchange currents, isobar configurations and to relativistic effects in different kinematical regions, serving as a benchmark for a test of present standard nuclear theory with effective degrees of freedom.     

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.     

Short-range effects in meson exchange currents for the radiative n-p capture of the deuteron

We show that a relatively complete calculation of meson exchange currents, including a short-range hadronic form factor for the pion exchange diagrams and the rho exchange currents can still explain the discrepancy between the theoretical and experimental cross section for the radiative n-p capture of the deuteron.     

SINGLE K MESON AND 2π, πρ EXCHANGES IN THE Λ-N INTERACTION

The purpose of this paper is to study, the A-N interaction by using meson exchange theory. We have considered three processes: ⅰ) Single K meson exchange, ⅱ) Box diagrams of 2π and πρ exchanges with a nucleon in the intermediate states, and ⅲ) Box diagrams of 2π and πρ exchange with an isobar in the intermediate states. For simplicity, two approximations are used in the calculation. First, the initial nucleon N and hyperon are considered to be at rest. Second, an average value is used for the energies of the intermediate states. The results are qualitatively consistent with experiments.     

Meson exchange currents in the Skyrme soliton model

The isovector meson exchange currents are investigated on the basis of the product ansatz in the Skyrme model. It is shown that the standard one-pion-exchange currents are correctly reproduced as a limiting approximation to more general expressions derived here. We discuss interesting consequences of the spatially extended structure of the nucleon, which is naturally incorporated in the present treatment. It is also pointed out that there exists a simple analytical relation between the isoscalar exchange current of the Skyrme model and the standard ρπγ exchange current.     

Mirror transitions and second class currents

Theoretical implications of observed mirror asymmetry in nuclear beta decay are examined, taking into account CP violation and internal symmetries. The second class induced tensor current is shown not to be responsible for the observed asymmetry, which must be the result of multinucleon or meson exchange effects. The result is general and holds for all values of momentum transfer. Possible two nucleon transitions are discussed. Mirror asymmetry in the Σ−Λ decay cannot be produced by SU(3) octet currents or single quark currents in the quark model even if such second class currents exist.     

Two-body deuteron photodisintegration above pion threshold with a dynamical treatment of δ degrees of freedom

The two-body photo break-up of the deuteron above pion threshold is calculated treating the final state interaction in a coupled channel approach with inclusion of Δ degrees of freedom. For the electromagnetic interaction nucleon, isobar and meson exchange currents are taken into account. Various observables (total and differential cross sections, γ-asymmetry and nucleon polarization) are influenced by the dynamical treatment of Δ degrees of freedom. Especially for the proton polarization it leads to an improved agreement with experiment. However, the situation for the shape of the differential cross section remains problematic.     

Relativistic Particle-Hole and Delta-Hole Excitations for Pion Propagator in Nuclear Matter

The relativistic particle-hole and delta-hole polarization insertions for pion propagator are calculated by using particle-hole-antiparticle representation of nucleon and delta propagators in nuclear matter. The short-range correlations between nucleon-nucleon, nucleon-delta and delta-delta are included via Landau-Migdal parameter g' in the random phase approximation. We calculate the dispersion relations for pions and find out that the damped pion condensation is removed by the short-range correlation and there is a long gap in the dispersion relation.     

A short history of nuclear magnetic moments and GT transitions

We summarize the history and our present understanding of nuclear magnetic moments and Gamow-Teller transitions.The roles of configuration mixing,meson exchange currents and relativistic effects are examined.Experimental evidence for the importance of tensor correlations is also discussed.     

n + p → d + γ in effective field theory

The radiative capture process n + p → d + γ provides clear evidence for meson exchange currents in nuclear physics. We compute this process at low energies using a recently developed power counting for the effective field theory that describes nucleon-nucleon interactions. The leading order contribution to this process comes from the photon coupling to the nucleon magnetic moments. At subleading order there are other contributions. Among these are graphs where the photon couples directly to pions, i.e. meson exchange currents. These diagrams are divergent and require the presence of a local four-nucleon-one-photon counterterm. The coefficient of this operator is determined by the measured cross section, σ^expt = 334.2±0.5 mb, for incident neutrons with speed |v| = 2200 m/s.     

A simple model for short-range correlations in nuclear matter

It is shown that Pauli blocking and short-range correlation effects, which strongly renormalize the bare nucleon-nucleon interaction in the medium, can be summarized in a correlation operator which has an extremely simple structure. The use of such a simple correlation operator leads to a form of Brueckner's reaction-matrix interaction which is very convenient for applications. The effective interaction introduced by the Jülich-Stony Brook group and extensively used in the study of the spin-isospin excitation modes in nuclei is derived from a meson exchange potential.     

Electromagnetic form factors of3He and3H

The electric and magnetic form factors of³He and³H are calculated with 3-nucleon wave functions obtained from the solution of Schrödinger equation with separable potentials of two different shapes which have already been employed in the coulomb energy calculation. The effect of important meson exchange corrections is evaluated and their dependence on the wave function studied. The form factors can depend rather sensitively on the nucleon form factors as well, and this dependence is studied by using two different parametrisations for the latter.     

Quark effects, meson-exchange currents and background in the d(e, e′p)Δ reaction

We have studied in detail the cross section for the d(e, e′p)Δ reaction leading to the emission of a fast nucleon and a Δ at rest, which has been advocated as a tool to investigate quark effects in nuclei. We find that ordinary meson-exchange currents mechanisms dominate the quark-exchange effects in the region of excitation of the Δ nd could be competitive at higher energies. Furthermore, at these higher energies, the small cross sections for the quark signal, together with the presence of a background about one order of magnitude bigger than the quark signal, make in any case the extraction of information about quark-exchange currents effects extraordinarily difficult.     

Effects of the HMT on Nucleon Collective Flows within BUU Transport Model

Within the framework of a semiclassical Boltzmann-Uehling-Uhlenbeck(BUU) transport model,the high momentum tail(HMT) effects of nucleon momentum distribution in the nucleus on the nucleon collective Bows are studied in semicentral Au+Au collisions.The HMT due to the isospin-dependent short-range correlations causes a smaller value of the collective Bows.We Bnd that the HMT effects on the nucleon collective Bows are remarkable at beam energy of 300 MeV/nucleon and become weak as the incident beam energy increases.The results indicate that for the collective Bow studies at intermediate energies,the HMT of nucleon momentum distribution in nucleus should be taken into account in transport models.     

Disorder and pseudogap in strongly correlated systems: Phase diagram in the DMFT + Σ approach

The influence of disorder and pseudogap fluctuations on the Mott insulator-metal transition in strongly correlated systems has been studied in the framework of the generalized dynamic mean field theory (DMFT + Σ approach). Using the results of investigations of the density of states (DOS) and optical conductivity, a phase diagram (disorder-Hubbard interaction-temperature) is constructed for the paramagnetic Anderson-Hubbard model, which allows both the effects of strong electron correlations and the influence of strong disorder to be considered. Strong correlations are described using the DMFT, while a strong disorder is described using a generalized self-consistent theory of localization. The DOS and optical conductivity of the paramagnetic Hubbard model have been studied in a pseudogap state caused by antiferromagnetic spin (or charge) short-range order fluctuations with a finite correlation length, which have been modeled by a static Gaussian random field. The effect of a pseudogap on the Mott insulator-metal transition has been studied. It is established that, in both cases, the static Gaussian random field (related to the disorder or pseudogap fluctuations) leads to suppression of the Mott transition, broadening of the coexistence region of the insulator and metal phases, and an increase in the critical temperature at which the coexistence region disappears.