Exchange Currents In Nuclei Research Articles

Chiral model of axial meson exchange currents in nuclei

The long-range part of the weak axial exchange currents (AXC) is considered in the framework of the chiral $\ensuremath{\sigma}$-$\ensuremath{\omega}$ model, which preserves basic symmetries of QCD and includes $\ensuremath{\sigma}$, $\ensuremath{\omega}$, and $\ensuremath{\pi}$ mesons. Linear and nonlinear realizations of the model are considered, and it is explicitly demonstrated that the way one splits relativistic many-body effects into AXC contributions and one-body relativistic corrections to the traditional nuclear axial current is model dependent. In the linear realization of the $\ensuremath{\sigma}$-$\ensuremath{\omega}$ model spatial two-body AXC are shown to be necessary to preserve the partial conservation of axial-vector current (PCAC) relation to order $O(1/M)$ in inverse nucleon mass. In the nonlinear realization of the model PCAC is satisfied for the one-body current identically. Indeed, it is shown that there are no AXC of order $O(1/M)$ in this realization, but one now has to include a new one-body relativistic correction of order $O(1/M)$ in the analysis to preserve PCAC for a single nucleon. A familiar result for axial exchange charge is reproduced in both models. The obtained correction terms are estimated in a simple unified analysis of electroweak processes with a few light nuclei.

Electron scattering and the few-nucleon system

After a short discussion of the formalism for electron scattering the information on the neutron charge form factor is extracted from the deuteron structure function. The cross sections for electrodisintegration of the deuteron at threshold and the behaviour of the trinucleon magnetic form factors are shown to provide evidence for the importance of meson exchange currents in nuclei. The trinucleon system in isospin space is discussed at the and of the lecture.

Non-nucleonic degrees of freedom in weak nuclear currents

Some of the key issues in the study of exchange currents in nuclei are surveyed. Particular emphasis is placed on the important connection between exchange-current effects and the effects of chiral symmetry in nuclei, a topic which is investigated with great intensity in the current literature.

Quark exchange currents in nuclei: Effective operator description

In this paper we introduce for the first time effective quark exchange current operators which can be used in nuclear structure calculations. To this end we first solve the equation of motion for a microscopic quark hamiltonian using the quark cluster model. We then construct the electromagnetic current operator on the quark level, i.e. the photon is coupled directly to the quarks. By eliminating the quark (internal) degrees of freedom, we derive an effective electromagnetic current operator on the nucleon level. A part of this effective current corresponds to the conventional impulse and meson exchange currents with vertex factors predicted by the quark model. In addition, this effective current contains new non-local and isospin-dependent terms which are generated by the Pauli principle on the quark level (quark exchange between nucleons). When we evaluate these quark exchange currents, we use harmonic-oscillator wave functions as nuclear wave functions including short-range correlations. We introduce these short-range correlations by solving the Bethe-Goldstone equation with our effective NN potential, which is derived from a microscopic quark hamiltonian. We investigate the role of these additional quark exchange currents in the magnetic moments and the elastic magnetic form factors of several closed-shell ± 1 nuclei, such as 15N, 17O, and 39K.

Exchange currents from chiral Lagrangians.

Exchange currents in nuclei are derived from chiral Lagrangians, and a justification is offered for the ``chiral filter hypothesis'' which seems to be supported by all presently available experimental data.

Topological-exchange-current contribution to the deuteron magnetic form factor.

In the chiral soliton (Skyrme) model all isoscalar electromagnetic currents, including the exchange currents in nuclei, are topological and uniquely determined by the current of the individual nucleons. This allows us to obtain a model-independent isoscalar exchange-current operator, which is used to study the magnetic form factor of the deuteron. The structure of the deuteron is incorporated by our evaluating the expectation value of the exchange-current operator with respect to the wave function obtained fom the Paris potential. The magnetic form factor of the deuteron is dominated by the exchange-current contribution and is in agreement with experimental data in the range 0

Mesonic exchange currents in nuclei

The standard meson exchange current derivation is discussed in view of the recent high momentum transfer electron scattering experiments for the two and three body systems and for heavier nuclei such as 51V. We emphasize the role of isovector magnetic transitions in the understanding of meson exchange current contributions and in particular for their connections with models of nucleon structure.

Exchange Currents in Nuclei

Exchange Currents in Nuclei

Baryonic excitations and meson exchange currents in nuclei

The rôle of mesonic and isobar degrees of freedom in nuclei and their effect on electron scattering and photonuclear reactions for low and medium energy physics are discussed. Specifically, two- and three-body systems are considered, where cleanest evidence for exchange current effects exists. Furthermore, exchange contributions to the two-photon amplitude for photon scattering are studied and results for deuterium are presented.

Exchange currents in nuclei, threshold pion photoproduction from nucleons, and the equivalence theorem

The experimental data for pion threshold photoproduction from nucleons are examined and are used to distinguish between models used to calculate pion-exchange currents in nuclei. The roles of the equivalence theorem and equivalence-breaking terms are stressed in both types of processes.

A consistent treatment of exchange currents in nuclei

A consistent treatment of two-particle exchange currents in nuclei in the framework of the one-boson-exchange-model for the nucleon-nucleon interaction is given. Relativistic effects are taken into account up to order ( P/ M) 2. The general formalism for the evaluation of matrix elements of two-body currents in many-nucleon systems is outlined. Possible applications to the two-nucleon system are briefly discussed.

Phenomenological Determination of the Importance of Exchange Currents in Nuclei

In this paper, it is shown that comparison between the magnetic dipole cross sections obtained from photon and electron disintegration experiments is a meaningful test for the determination of the importance of the exchange currents in nuclei. The proof is obtained by explicit use of the Foldy-Osborn invariants. A detailed discussion is given of the kinematical situation in which the test should be applied.

Dispersion theory and exchange currents in nuclei

In this work the pion exchange current contribution to the form factor of model deuteron is discussed. Starting from the idea of the Feynman diagram of exchange current, this contribution is then separated out from the rest of the direct current in dispersion theory in the lowest (4-th) order of approximation. Higher order contributions may not have the simple meaning of being exchange currents and are not, in general, separable in dispersion theory. A practicable method of evaluating these pion exchange current contributions to the form factors of light nuclei, such as2H,3H and3He, is suggested.

Exchange Currents in Nuclei

The existence of exchange forces between nucleons, that is, of forces requiring the exchange of identity of a proton and neutron during an interaction between them, implies that electric charge must be transferred between the neutron and proton and therefore that electric currents flow in the intervening space between the particles.

The Phenomenological Theory of Exchange Currents in Nuclei

As was first pointed out by Siegert, the existence of exchange forces in nuclei implies the existence of accompanying exchange currents. Sachs has calculated an expression for these, by making the Hamiltonian containing exchange potentials gauge-invariant, and has applied it to the calculations of exchange magnetic moments in ${\mathrm{H}}^{3}$ and ${\mathrm{He}}^{3}$. The Hamiltonian obtained by Sachs is not the most general admissible one. More generally, the exchange current density is found to depend on a vector function whose irrotational part is completely determined by gauge-invariance but whose solenoidal part is arbitrary except for the requirements (following from conditions of translational invariance and symmetry in all nucleons on the Hamiltonian) that it be translationally invariant and antisymmetric under the exchange of the spin and space coordinates of each pair of nucleons. Making use of these conditions on the Hamiltonian, the explicit form of the dependence of the solenoidal part of the exchange current upon the spin and isotopic spin coordinates of the nucleons has been derived. In the resultant exchange moments, the irrotational part leads to the expression obtained by Sachs, while the solenoidal term contribution contains the spin operators of the nucleons in particular combinations, together with arbitrary functions of the nucleon separation. Villars' exchange moment expression, as obtained from meson theory, is included as a special case and hence the exchange contributions to the moments of ${\mathrm{H}}^{3}$ and ${\mathrm{He}}^{3}$ are explicable on a phenomenological basis, contrary to the results obtained in Sachs' special case. The generality and significance of the results are discussed in relation to the various meson theories.

Phenomenological Theory of Exchange Currents in Nuclei

On the assumption that the interaction between nuclear particles involves a space exchange operator, it is shown that an addition must be made to the conventional current density for the nucleons in order to establish the equation of continuity within the nucleus. A general expression is found for this exchange current and the corresponding exchange magnetic moment. This phenomenological theory has application to the calculation of magnetic moments of nuclei and to the calculation of transition probabilities for the absorption and emission of radiation by nuclei. In this paper, application is limited to the exchange moments of ${\mathrm{H}}^{3}$ and ${\mathrm{He}}^{3}$. It is found that the exchange moments are six or seven times too small to account for the observed moments. In view of results obtained by Villars, it is concluded that the important contributions to the magnetic moment are directly related to the properties of the field (meson field) which describes the nuclear interaction, so the exchange moment may be of use for obtaining direct information concerning the nature of this field.