Meson exchange effects on electron scattering form factors in 12C

Abstract

The role of meson-exchange currents in the excitation of nuclear states by electron scattering is investigated in the favourable case of 12C. The description uses the non-covariant perturbative model based on the expansion of the mesic S-matrix in the interaction representation. Only the one-pion-exchange contributions to the long-range part of the two-body electromagnetic interaction are retained. When coming to the derivation of the multipole operators, the two-body local current densities are expanded around the c.m. of the pair of nucleons. The approximation is introduced chiefly to simplify the practical calculations, but it also allows the model to fulfil the requirement of charge conservation. Application of the model is made to the strongly excited isospin-one levels lying above 14MeV excitation in 12C. The electron scattering form factors are calculated with wave functions of the single particle-hole model and compared with data for momentum transfers extending up to 700 MeV/ c. The exchange correction is found to be small for low values of the transfer but to grow substantially as the transfer increases; so much as to overcome the contribution of the one-body operators. The effect is always an increase of the form factors with a similar dependence on transfer as the one-body form factors. Comparison of the results for the impulse form factors with other recent theoretical calculations show a reasonable degree of model independence. The spin-orbit relativistic corrections to the one-body currents are also examined but are found to play a negligible role, particularly in the transverse interaction.