(e, e′π) reaction on light nuclei and spin-isospin strength distribution effects

We study threshold pion electroproduction on light $T=0\mathrm{}$ nuclei. The probe is especially interesting because it mixes longitudinal (spin component parallel to momentum transfer, $\overrightarrow{\sigma }·\overrightarrow{\mathrm{k}}$ coupling) and transverse ($\overrightarrow{\sigma }\times \overrightarrow{\mathrm{k}}$ coupling) channels. Using the local density approximation for the treatment of possible nuclear spin-isospin mode enhancements, we calculate cross sections for both exclusive and inclusive measurements. For discrete nuclear levels we study differential cross sections of various pionlike levels, and these are compared with a nonpionlike excitation. The cross sections for exclusive excitations are extremely small except for forward angles. The inclusive cross sections, achieved by summing over all nuclear states, are considerably larger since these include the quasifree excitation. All cases are studied as a function of $g^{\prime }$, the Migdal spin-isospin parameter. We find a strong influence of kinematical conditions (especially the energy transfer) and of nuclear density on enhancement phenomena.