| Quantum Chromodynamics (QCD) is the theoretical key to study the structure of the nucleon. However, it is difficult to use, particularly in the non-perturbative domain. This work uses electromagnetic interactions to gain new theoretical insights into the non-perturbative chromodynamic structure of the nucleon. This is done in two ways. The first method adopted is the Lattice Gauge Theory to study the spin structure of the nucleon and try to answer the question, “Where does the nucleon spin come from ?”; To date, theoretically, we do not have a comprehensive understanding about the nucleon spin structure. Experimental findings reveal that quark spin contributes only a small fraction to the proton spin. However, there are no data which can conclusively tell us about the contribution of gluons and the orbital angular momentum of quarks. Through lattice calculations, we have studied the total angular momentum contributions from quarks, thereby deducing the quark orbital angular momentum, and thus enabling prediction of the gluon contribution to the proton spin. This work suggests that the total angular momentum content of the quark in the proton is about 60%, and the remaining 40% is attributable to the glue. In addition, it is found that in the quark sector, about 35% of the proton spin originates from the quark orbital angular momentum and the rest (25%) is due to quark spin.; The second subject that we explore is the electromagnetic excitation of the nucleon to its resonance states, particularly the N*(1520). Using an effective Lagrangian approach, recently obtained polarization data on eta photoproduction are shown to be very useful in yielding model-insensitive constraints on the electrostrong parameters for the excitation and decay of the N*(1520) resonance. An important quantity, , characteristic of photo- and electro-excitation of this resonance, is extracted, which provides a critical test for QCD-inspired hadron models. For electroproduction of the eta meson, recent experimental data from Jefferson Laboratory (J-Lab) are analyzed, and a modest sensitivity to the N*(1535) is found. It is then demonstrated that a recently completed double polarization experiment (J-Lab) is sensitive to the N*(1520) resonance, which may lead to the possibility of accurate extraction of the N*(1520) parameters.; Findings in this study illuminate our knowledge about QCD. |
