Factorization of the Lepton-Nucleus Cross Section and Spectral Function Formalism*

In the impulse approximation regime, factorization of the nuclear cross section provides a unified and consistent framework for extracting the information on target structure and dynamics from the data, thus allowing to assess the validity of nuclear models. This scheme,  whose basic element  is the nuclear spectral function, has been remarkably successful in explaining electron-nucleus scattering data at beam energies around and above 1 GeV. Being an intrinsic property of the target ground state, the spectral function determines the cross section in any channels, for both charged lepton and neutrino beams. In addition, being trivially related to the two-point Green's function, it allows to pin down the effects of nucleon-nucleon correlations in a fully model independent fashion. I will briefly discuss the potential and limitations of the spectral function formalism, and its extensions to include effects---such as final state interactions and coupling to meson-exchange currents---not taken into account at the impulse approximation level.