Electromagnetic Interactions In Nuclei Research Articles

Consistent three-nucleon forces in the nuclear many-body problem.

Describing an assembly of an infinite number of nucleons in interaction via a two-body potential as a nonrelativistic many-body problem in the first place, we envisage corrections to this picture due to suppressed degrees of freedom at the level of the two-body potential. At variance with relativistic many-body theory, the solution of the nonrelativistic problem with a two-body potential only is sufficiently under control at present so that evaluating corrections in this framework is of particular interest. These corrections come primarily from additional three-body forces either due to finite-density effects (Pauli blocking of fermions) or are of genuine origin: relativistic dynamical processes and effects from the intrinsic structure of the nucleon. Recalling the successful treatment of electromagnetic interactions in nuclei in terms of meson-exchange currents, we establish novel consistency requirements between the initial two-body force and the well-identified residual three-body force. In this way no new parameters enter in the three-body force, save for the controversial mass of the fictitious scalar ``\ensuremath{\sigma}'' meson. We show further that the nucleon-antinucleon pair term required in the analysis of meson-exchange currents has a genuine three-body counterpart resulting from time-ordered diagrams containing a single Z branch. Its contribution to the energy per particle is repulsive and varies with a high power of the density. Thereby we obtain the important saturating effect present in relativistic mean-field approaches. We envisage next the role of the first radial nucleon resonance ${N}^{\mathrm{*}}$((1/2,1)/2) (Roper resonance) in inducing a specific three-body force. The meson-nucleon-Roper coupling constants and form factors are evaluated in a relativistic quark model. Gathering all self-consistent corrections to the binding energy per particle of infinitely many nucleons, we find that the final equation of state is solely governed by the density dependence of medium corrections to the free \ensuremath{\sigma}-meson mass. We discuss a first attempt to extract this density dependence from an empirical equation of state.

Weak and Electromagnetic Interactions in Nuclei

Weak and Electromagnetic Interactions in Nuclei

Semileptonic weak and electromagnetic interactions in nuclei: Recoil polarization in muon capture

An analysis of the polarization of the recoiling nucleus following the capture of polarized muons by nuclei is performed. New general expressions for abitrary nuclear spin are obtained in terms of the same reduced matrix elements which govern inelastic electron scattering and β -decay. As an application we consider the A = 12 system and study uncertainties in the nuclear structure by using different sets of one-body density matrices. With the canonical values of the weak form factors (i.e. absence of second-class currents) we achieve a fairly good agreement with the experimental data including the inelastic form factor at high momentum transfers and the recently measured average 12B polarization. Implications Of the new corrected value of the average polarization on weak form factors and nuclear structure are discussed.

Semi-leptonic weak and electromagnetic interactions in nuclei:: Parity violations in electron scattering and weak neutral currents

A general result for the difference in differential cross sections for electron scattering between any two nuclear levels with incident longitudinally polarized right- and left-handed electrons is derived. This difference must involve the parity-violating weak interaction at least linearly and can be used to study weak neutral currents as pointed out by Feinberg. A V — A structure for the weak neutral currents is assumed with a semi-leptonic current-current interaction, and the electromagnetic interaction is treated in the one-photon-exchange approximation. The result is expressed in terms of the same set of reduced matrix elements of the multipoles of the nuclear currents which govern all electromagnetic and weak transitions between these levels. A previously developed unified analysis of semi-leptonic weak and electromagnetic interactions in nuclei which determines one-body transition densities, including their spin and spatial dependences, through electron scattering provides nuclear transitions to serve as known analyzers in testing the structure of this part of the weak interaction. Examples are given using Weinberg's model of the weak neutral currents. Feinberg's result for elastic scattering from spin-0 nuclei is rederived and two new examples using previously determined one-body densities are discussed : the 1 +0 → 0 +1 (3.56 MeV) transition in 6Li and the 0 +0 → 1 + 1(15.1 MeV) transition in 12C.

Semi-leptonic weak and electromagnetic interactions in nuclei with application to 16O

Electron scattering is shown to provide a basis for analyzing semi-leptonic weak interactions in nuclei essentially independent of detailed models of nuclear wave functions. Application is made to 16O. One result is the identification of a second-class-current detector.

Microscopic Theory of Effective Operators for Electromagnetic Interactions in Nuclei

Effective operators for valence (open-shell) nucleons only are constructed, for electromagnetic interactions in nuclei. Such operators are calculated in terms of excitations and deexcitations of particle-hole pairs of the oore nucleons. The nuclear force involved is a realistic; nucleon-nucleon potential (Yale-Shakin). The theory is applied to the Sn region described in terms of five valence neutron subshells and of all the core and open subshells of both the neutrons and protons between the magic numbers 8 and 126. The effective quadrupole charge matrix has all the elements of about the same order of magnitude,thus giving a first partial support to the concept of a oonstant neutron effective charge. Calculations of observables are performed with the states of the even tin isotopes described in terms of the twoand fourquasiparticle Tamm-Dancoff theories involving explicitly only the valence neutrons. Results are presented for the B(E2, 2, * 0 . ) t the quadrupole moment Q ( 2 ) , the gyromagnetic factor g and the inelastic electron-scattering form factors for the 2+ and 3r 116 — XI states of Sn . Except for the last (3-* ) form factor, a good seraiquantitative agreement with all the corresponding experimental data is obtained. The reported calculations involve no ad hoc adjustable parameter.

Effective Operators of Electromagnetic Interactions in Nuclei and Realistic Nucleon-Nucleon Potentials

Electromagnetic interactions in nuclei are described in terms of effective operators for valence (open-shell) nucleons only. The effective operators are calculated theoretically from a realistic nucleon-nucleon potential (Yale-Shakin) in terms of excitations and de-excitations of particle-hole pairs of the core nucleons. The theory is successfully applied to ${\mathrm{Sn}}^{116}$ whose states are described in terms of the two- and four-quasiparticle Tamm-Dancoff theories involving valence neutrons only.

Effective Operators of Electromagnetic Interactions in Nuclei and Realistic Nucleon-Nucleon Potentials

Electromagnetic interactions in nuclei are described in terms of effective operators for valence (open-shell) nucleons only. The effective operators are calculated theoretically from a realistic nucleon-nucleon potential (Yale-Shakin) in terms of excitations and de-excitations of particle-hole pairs of the core nucleons. The theory is successfully applied to ${\mathrm{Sn}}^{116}$ whose states are described in terms of the two- and four-quasiparticle Tamm-Dancoff theories involving valence neutrons only.