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

We have measured a new electron scattering observable, the “fifth” structure function, ${\mathrm{f}}_{01}^{\prime }$, 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. ${\mathrm{f}}_{01}^{\prime }$ arises from interference between reaction channels and provides an additional means for their disentanglement. In the quasielastic ${\mathrm{C}}^{12}$(e→,e′p) measurements reported here, ${\mathrm{f}}_{01}^{\prime }$ is driven by the interference of the direct knockout and rescattering amplitudes.