Longitudinal and transverse quasielastic response functions of light nuclei

The ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ longitudinal and transverse response functions are determined from an analysis of the world data on quasielastic inclusive electron scattering. The corresponding Euclidean response functions are derived and compared to those calculated with Green's function Monte Carlo methods, using realistic interactions and currents. Large contributions associated with two-body currents are found, particularly in the ${}^{4}\mathrm{He}$ transverse response, in agreement with data. The contributions of the two-body charge and current operators in the ${}^{3}\mathrm{He},$ ${}^{4}\mathrm{He},$ and ${}^{6}\mathrm{Li}$ response functions are also studied via sum-rule techniques. A semiquantitative explanation for the observed systematics in the excess of transverse quasielastic strength, as function of mass number and momentum transfer, is provided. Finally, a number of model studies with simplified interactions, currents, and wave functions are carried out to elucidate the role played, in the full calculation, by tensor interactions and correlations.