Isobar configurations in closed shell nuclei

Explicit expressions for one- and two-N¹ isobar configurations in closed shell nuclei have been derived in the impulse approximation and applied to ⁴He and ¹⁶O. Conventional transition potentials with π- and ?-exchange have been used and the influence of potential regularization and short-range correlations have been studied. The dominant isobar configurations are found to be the one- and two-Δ components and originate from two-nucleon pairs in a relative S-state. The probability for finding a nucleon as a Δ-isobar is about 4.5% in ⁴He and about 3.2% in ¹⁶O. These numbers should be considered as upper limits due to uncertainties in off-shell effects of the transition potentials.     

Polarizationeffects in electron-electron and positron-electron scattering

The polarization-asymmetries in the cross-sections for electron-electron and positron-electron scattering are investigated numerically in first order perturbation theory in the range from 1 keV to 10 GeV for all angles with regard to polarization-measurements.