Before the discovery of Quantum Chromodynamics, the parton model provided a correct description of scaling violations in deep inelastic leptoproduction. Today, we understand the QCD basis for this model. Using the light-cone operator product expansion, one can organize contributions to moments (integrals) of structure functions in deep inelastic scattering according to their "twist", defined as the difference between an operator's canonical dimension and its spin. The anomalous dimensions of leading twist operators correspond to moments of the splitting functions that appear in DGLAP evolution of parton distribution functions. Physically, the leading twist/parton model contributions correspond to those that arise from the incoherent scattering of leptons from point-like constituents of the target.

At sufficiently high-energy, the leading twist contributions dominate. However, as the Q² of the scattering process is lowered, power corrections associated with higher twist contributions may become important. Physically, these contributions correspond to effects that arise from correlations between the quarks and gluons inside the target during the scattering process - correlations that are absent from the leading twist terms. If the higher twist contributions can be observed and correctly analyzed, they can provide a new look into the substructure of protons and neutrons, providing interesting information about the internal landscape of the nucleons beyond the view we already have from the parton model description.

Recently, data collected by the CLAS collaboration at Jefferson Laboratory has uncovered what appear to be signatures of higher twist effects in structure function moments. The data present puzzles that call for a theoretical resolution. NPAC theorists and their collaborators have undertaken a study of these higher twist effects, seeking to interpret them rigorously within the context of QCD. The long term goal is to be able to extract from the data higher-twist operator matrix elements that can be compared with computations using lattice QCD. We are also studying the implications of higher twist contributions for parity-violating deep inelastic scattering, in view of future experiments that are planned for the Jefferson Laboratory after the 12 GeV upgrade. We hope to determine whether these parity violation experiments can provide a complementary window on the higher twist structure of the proton.