First Measurement of the Imaginary Part of the Transverse-Longitudinal Nuclear Response.

We have measured a new electron scattering observable, the ''fifth'' structure function, f01', which is the imaginary part of the transverse-longitudinal interference response. Its observation requires a longitudinally polarized beam and coincident, out-of-plane particle detection. f01' arises from interference between reaction channels and provides an additional means for their disentanglement. In the quasielastic C-12(e over arrow pointing right, e'p) measurements reported here, f01' is driven by the interference of the direct knockout and rescattering amplitudes.