Currents In Nuclei Research Articles

Chiral electroweak currents in nuclei

The development of the chiral dynamics based description of nuclear electroweak currents is reviewed. Gerald E. (Gerry) Brown’s role in basing theoretical nuclear physics on chiral Lagrangians is emphasized. Illustrative examples of the successful description of electroweak observables of light nuclei obtained from chiral effective field theory are presented.

Large-scale nuclear structure calculations for spin-dependent WIMP scattering with chiral effective field theory currents

We perform state-of-the-art large-scale shell-model calculations of the structure factors for elastic spin-dependent WIMP scattering off 129,131Xe, 127I, 73Ge, 19F, 23Na, 27Al, and 29Si. This comprehensive survey covers the non-zero-spin nuclei relevant to direct dark matter detection. We include a pedagogical presentation of the formalism necessary to describe elastic and inelastic WIMP-nucleus scattering. The valence spaces and nuclear interactions employed have been previously used in nuclear structure calculations for these mass regions and yield a good spectroscopic description of these isotopes. We use spin-dependent WIMP-nucleus currents based on chiral effective field theory (EFT) at the one-body level and including the leading long-range two-body currents due to pion exchange, which are predicted in chiral EFT. Results for all structure factors are provided with theoretical error bands due to the nuclear uncertainties of WIMP currents in nuclei.

Spin-dependent WIMP scattering off nuclei

Chiral effective field theory (EFT) provides a systematic expansion for the coupling of WIMPs to nucleons at the momentum transfers relevant to direct cold dark matter detection. We derive the currents for spin-dependent WIMP scattering off nuclei at the one-body level and include the leading long-range two-body currents, which are predicted in chiral EFT. As an application, we calculate the structure factor for spin-dependent WIMP scattering off 129,131Xe nuclei, using nuclear interactions that have been developed to study nuclear structure and double-beta decays in this region. We provide theoretical error bands due to the nuclear uncertainties of WIMP currents in nuclei.

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.

New version of Moscow NN model

Two-phase NN-interaction model with a partial microscopic substantiation is presented. In contrast to other hybrid models our approach includes two mutually orthogonal phases or channels: multiquark channel and meson-exchange channel. The one-channel Moscow potential model has been shown to appear as a phase-shift equivalent NN-potential with extra bound states. Some general consequences for few-nucleon systems and meson-exchange currents in nuclei are discussed.

Strange neutral currents in nuclei.

We examine the effects on the nuclear neutral current Gamow-Teller (GT) strength of a finite contribution from a polarized strange quark sea. We perform nuclear shell model calculations of the neutral current GT strength for a number of nuclei likely to be present during stellar core collapse. We compare the GT strength when a finite strange quark contribution is included to the strength without such a contribution. As an example, the process of neutral current nuclear deexcitation via \ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{} pair production is examined for the two cases. \textcopyright{} 1996 The American Physical Society.

Chiral Lagrangian approach to exchange vector currents in nuclei

Exchange vector currents are calculated up to one-loop order (corresponding to next-to-next-to-leading order) in chiral perturbation theory. As an illustration of the power of the approach, we apply the formalism to the classic nuclear process $n+p\rightarrow d +\gamma$ at thermal energy. The exchange current correction comes out to be $(4.5 \pm 0.3)$ \% in amplitude giving a predicted cross section $\sigma= (334\pm 3)\ {\mbox mb}$ in excellent agreement with the experimental value $(334.2\pm 0.5)\ {\mbox mb}$. Together with the axial charge transitions computed previously, this result provides a strong support for the power of chiral Lagrangians in nuclear physics. As a by-product of our results, we suggest an open problem in the application of chiral Lagrangian approach to nuclear processes that has to do with giving a physical meaning to the short-range correlations that play an important role in nuclei.

Mr. Maxwell's journey into hadron country (electromagnetic currents in nuclei)

In these lectures an introduction is given into the main features of nuclear structure investigations by photoabsorption and electron scattering. After a brief review of the general structure of the electromagnetic interaction, the gauge conditions and low-energy theorems for the electromagnetic interaction operators are discussed. The various theoretical schemes for incorporating subnuclear degrees of freedom either as effective operators like the pion exchange current or explicitely like in the model of nuclear isobar configurations are reviewed. At the end, a few specific experiments are discussed as an illustration of what can be learned from such experiments.

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.

Neutron densities from muon capture

We show that, because of Pauli blocking and renormalization of the weak currents in nuclei, the muon capture rates are rather sensitive to the neutron distributions. We also show that, because of intrinsic theoretical uncertainties, neutron radia cannot be determined with precision but some reasonable limits can be given. However, the ratio of capture rates in different isotopes serves to determine the neutron radii of the isotopes provided the neutron density distribution for one of them is known.

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.

Axial currents in nuclei in the framework of the sigma model

The renormalization of the axial-vector coupling constant in nuclear matter due to the scalar 〈 Ψ Ψ〉 condensate suppression is considered in the framework of the modified sigma model. The nucleon effective mass and the pion decay constant in nuclear matter are also calculated.

Convection currents in nuclei in a relativistic mean-field theory.

Isoscalar convection currents for nuclei with one valence particle or hole outside a closed-shell core are studied in the mean-field (Hartree) approximation to the Walecka model. The currents are constructed using nucleon wave functions computed in the self-consistent potentials of the spherical core. Approximate self-consistency for the full odd-A nucleus is imposed by using linear response theory to correct the core wave functions for the effects of the valence nucleon. At low q, this core response cancels the enhancement of the isoscalar current that arises from the reduced effective mass of the valence nucleon so that magnetic moments are close to the Schmidt predictions. In contrast, particle-hole mixing in the core response enhances the single-particle current further at higher q. Calculations of magnetic moments and elastic magnetic scattering in light nuclei are presented to illustrate these results. The relationship of these calculations to fully self-consistent mean-field solutions of odd-A nuclei and to the linear response function in nuclear matter is discussed.

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.

Induced Weak Currents in Nuclei

The vector--axial vector description of weak charged currents in nuclear theory is reviewed. Tests of conserved vector current, PCAC, and second--class currents are discussed. (AIP)

Axial currents in nuclei and the skyrmion size.

Combining the skyrmion picture of the nucleon and the axial Ward identities in nuclear medium, we show that in a baryon-rich environment, the coupling constants ${\mathrm{g}}_{\mathrm{A}}$ and ${\mathrm{f}}_{\mathrm{\ensuremath{\pi}}}$ are fundamentally quenched and the rms size of the nucleon consequently enhanced..AE

Induced Weak Currents in Nuclei

Induced Weak Currents in Nuclei