Qian Wan

In this thesis, a calculation of all nucleon electromagnetic form factors is reported. The calculation is done in a two-component model, where the the electromagnetic couplings of baryons are assumed to have two components: direct coupling to the internal qqq structure and a coupling through meson cloud qq¯ to qqq. The meson cloud is parameterized as the vector mesons rho, o and &phis;. The algebraic structure of baryons is Ur(7)⊗ SUf(3)⊗SUs(2)⊗ SUc(3), where the index r,f,s,c denotes spatial, flavor, spin and color. The spatial part group structure of U(7) is obtained through the algebraic methods. It is assumed that the spatial part of the baryon follows the distributed collective (string-like) model. The transition Hamiltonian is constructed based on the elementary emission model. Compared to other models, our approach has the following advantages: All form factors are obtained simultaneously in a unified algebraic model, thus, very few parameters are needed to produce the results on a large set of resonances. The calculation is done in an explicit analytical form and thus is transparent. In this thesis, we analyze experimental data on various baryon states using the two-component model, in particular, recent high precision data collected at JLab and other new facilities. It appears that the two-component model is able to explain many new interesting features of these data, such as the violation of scaling invariance of elastic form factors, the deviance of the data on N-Delta transition from the constituent quark model, etc. It is evident that vector mesons play a vital role in the electromagnetic transitions. According to our results, the introduction of isospin symmetry breaking not only reproduces physical values of mu p and mun, but also improves the calculation on the neutron form factors without deteriorating the results on proton. Relativistic corrections appear to play an important role, which has an across-the-board improvement on the results, especially in the low Q2 region. The mixing effect by the tensor interaction is also considered to explain the data on selected resonances.