Longitudinal structure of the proton and neutron

Lepton scattering has been utilized for more than four decades to study the substructure of protons and neutrons, both free and bound in nuclei. The $F_2$ structure function extracted from such experiments has now been determined over many orders of magnitude in both Bjorken $x$ and the 4-momentum transfer squared, $Q^2$, and such data have been an invaluable tool for the testing and study of Quantum Chromodynamics (QCD) in the perturbative regime. In addition, new precision data on $F_2$ from Jefferson Lab in the region of the nucleon resonances has opened up many new studies of the transition from perturbative to non-perturbative QCD. This includes the nature of quark-hadron (Q-H) duality, in which the resonances seem to average to a smooth scaling curve, similar to the that of deep inelastic scattering. In contrast, the longitudinal structure function, $F_L$, has been measured with much poorer precision and over a much more limited kinematic range. This is due to both the high precision required and the need for measurements at fixed $x$ and $Q^2$ with multiple beam energies for the separation of longitudinal and transverse structure. Such measurements are critical for a full picture of nucleon structure. For instance, in deep inelastic scattering $F_L$ is directly sensitive to the gluon content of the nucleon, unlike $F_2$, which is only sensitive through pQCD evolution. We will present the current experimental status of $F_L$ for both free protons and from nucleons in nuclei and discuss some of the physics which can be addressed with such measurements. In particular, studies of Q-H duality and the determination of the structure function moments will be discussed.