Out-of-plane measurements of the fifth response function of the exclusive electronuclear response
The first measurements of ${f}_{\mathrm{LT}}^{\ensuremath{'}},$ known as the fifth response function, have been made for the ${}^{2}\mathrm{H}(\stackrel{\ensuremath{\rightarrow}}{e}{,e}^{\ensuremath{'}}p)$ and ${}^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{e}{,e}^{\ensuremath{'}}p)$ reactions. This response is directly related to the imaginary part of the interference between the transverse and longitudinal nuclear electromagnetic currents. Its observation requires longitudinally polarized electron beams and out-of-plane detection, the latter made possible by the newly developed out-of-plane spectrometer system. The initial measurements were made by using a 560-MeV polarized electron beam and quasielastic kinematics at ${Q}^{2}=3.3 {\mathrm{fm}}^{\ensuremath{-}2}.$ The development of the methodology for out-of-plane physics, and the analysis of the data from the initial experiments are described in detail. The measured fifth response and the related asymmetry in the coincidence cross section are in agreement, albeit with large statistical errors, with the theoretical predictions. Future extensions of the out-of-plane program are also discussed.