MESON-EXCHANGE CURRENTS IN QUASI-ELASTIC ELECTRON-SCATTERING FROM C-12 AND CA-40 NUCLEI

Quasi-elastic transverse responses in C-12 and Ca-40 have been calculated considering both one- and two-body nuclear currents. In the two-body current we have taken into account, in addition to the contact and pion in flight terms, also diagrams including the contribution of the DELTA isobar. In the evaluation of the transverse response both one-particle one-hole (1p1h) and 2p2h states have been considered. Our calculations show that the contribution of the meson-exchange currents (MEC) to the 1p1h channel reduces the transverse response. This effect is partially compensated in the peak region by the 2p2h response. A study of the relative importance of the various pieces contributing to the response function is presented. Our 1p1h results (including MEC) are in agreement with a Fermi gas calculation performed with an effective value of the Fermi momentum. The comparison with the experimental data is done after the inclusion of the final state interaction