D.O. Riska

Pionic disintegration of the deuteron

Abstract 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 ϱ-meson rescattering calculated from phenomenological Lagrangians. The role of the ϱ-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.

PION EXCHANGE CURRENTS IN ELASTIC ELECTRON DEUTERON SCATTERING

The effects of pion-exchange pari and recoil currents on the electromagnetic form factors of the deuteron are calculated. Both exchange currents give significant contributions to the charge form factor and, despite some cancellation, their net effect is appreciable for q2 ⩾ 10 fm−2.

The magnetic form factor of 3He

Abstract The behavior of the magnetic form factor of 3 He for momentum transfers q 2 −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 q 2 > 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.

PIONIC DISINTEGRATION OF THE DEUTERON

Abstract A simple model for π + d → pp in the resonance region based on two-body absorption by pion and ϱ-meson rescattering through the Δ 33 resonance is presented. Without introduction of free parameters or form factors the calculated total cross section is in fair agreement with experiment.

The effect of pion exchange currents on the charge form factor of 4He

Abstract Pion exchange currents give large contributions to the charge form factor of the α-particle in the momentum transfer region around the observed minimum and secondary maximum. The effect of the exchange currents is to bring the calculated form factor closer to the observed values.

The reactive two-body part of the pion-nucleus optical potential

Abstract We calculate the reactive component of the two-body contribution to the pion-nucleus optical potential from a two-nucleon pion absorption mechanism that predicts the total cross section and angular distributions for π+d → pp very well. At threshold the calculated absorptive parts explain most of the values obtained from pionic atom level widths, whereas the dispersive parts, which are very sensitive to wave function correlations are considerably more attractive than what the conventional phenomenological parameters would suggest.

Charge symmetry breaking interactions and coulomb energy differences

Abstract A detailed study of the possibility that the long standing discrepancies between theory and experiment in the values of Coulomb displacement energies have their origin in the charge asymmetry of the nuclear force is presented. A review of the present status of the theory of Coulomb displacement energies of mirror states is given. We have constructed a phenomenological charge asymmetric potential that can remove the Coulomb energy discrepancies for several mirror nuclei yet having only minimal effect on the 1 S 0 nucleon-nucleon scattering length. The form of this potential is however not compatible with the contributions due to charge asymmetric meson exchange processes that have been considered in the literature. We assume that these meson exchange effects can be reasonably described, up to strength factors, by a sum of one-pion, one-scalar-meson (σ) and one-vector-meson exchange potentials. The need for some additional type of charge asymmetric effect, e.g. three-body forces, is pointed out.

The absorptive P-wave pion-nucleus optical potential

Abstract We derive the absorptive part of the P-wave pion-nucleus optical potential from a two-body model for the absorption mechanism which involves rescattering of a pion and ϱ-meson through a Δ 33 resonant state. The model gives an adequate explanation for the fundamental π + d → pp reaction cross section and leads to values for the optical potential parameter which are in fair agreement with those obtained from pionic atom level widths.

Pion exchange currents and nuclear charge form factors

Abstract The effect of the pion exchange current due to the Born term in the photoproduction amplitude on the charge form factors and distributions of 4 He, 16 O and 40 Ca is calculated in the simple harmonic oscillator shell model. The exchange current contribution is most notable in the case of the α-particle while it is rather unimportant in 16 O and 40 Ca. The contribution to the nuclear mean square radius is small (≈ 0.03 fm 2 ) and roughly independent of the mass number.

The effect of the nuclear medium on s-wave pion absorption

Abstract It is shown that the effects of the nuclear medium on the pion propagator and the renormalization of the πNN interaction, described in terms of excitation of virtual isolar-hole pairs can enhance the predicted absorption rates for pions at threshold to values close to the empirical ones.