Exotic Hadrons

Exotic hadrons are striking predictions of QCD (quantum chromodynamics) beyond the quark model. Recent experimental advances in hadron spectra have given rise to much theoretical interest in exotica. This thesis explore several aspects of. the exotic hadron problem, which mainly involves the possible pentaquarks and 0⁺⁺ mesons, in order to get some insight into understanding hadron structure and dynamics. A number of hadronic observables are calculated systematically using different methods based on the nonlinear chiral Lagrangian formalism. This thesis is organized as follows.In Chapter 1 we start with the necessary general knowledge as well as the basic motivations and ideas for our exotic study. After a brief review of the underlying QCD symmetries with a focus on the spontaneous chiral symmetry breaking and its nonlinear realization, we move on to discuss about the general scheme of constructing a low energy effective field theory based on the nonlinearσ-model.In Chapter 2 we first give a general description of the concept and formalism development of the chiral soliton models and quark models. Subsequently, we present a detailed analysis of the standard SU(3) Skyrme model, which are extended in Section 2.3.1 by incorporating the vector meson fields as massive external gauge fields in the nonlinearσ-model. We then look for topological soliton solutions of the model and embark on detailed investigations of the static properties of the baryons in the rigid rotator approximation. In Chapter 3 the extended Skyrme lagrangian is employed to evaluate the static properties of the low-lying J=1/2 pentaquarks, with the mixing representations up to35 and/or (35|-). The exotic and normal mass splittings are calculated in perturbationtheory with corrections up to third-order in energy and second-order in wavefunction, and the widths for various hadronic decay modes of the pentaquark states along with that of the decuplet are derived in detail via the axial-vector current approach. The resultant masses and decay widths are compared with the experimental data available as well as the relevant predictions in other pictures. We recognize that the higher order calculations are not negligible, and our analysis provides some valuable clues to the physical mechanisms. What's remarkable is that once the conventional baryon properties are fit other states are predicted without any more adjustable parameters, and that both symmetry-breaking and decay operators are used in full. In addition, we list some formulations for the future calculation of the electromagnetic observables, such as magnetic momentums, magnetic transition moments and the squared electric and magnetic radii.The formulated iagrangian of Section 2.3.1 is further extended by explicit inclusion of putative scalar mesons and the possible electromagnetic gauge fields as well as trilinear SVV (scalar-vector-vector) vertices are also introduced in Chapter 4 arid applied to study the mass spectra and the decay properties of 0⁺⁺ states below 2 GeV. Assuming that f₀(980), a₀(980),K₀^*(1430), and f₀(1500) comprise an SU(3) nonet with respect to the current experiments, we make a detailed prediction about the static properties of the 0⁺⁺ mesons. The substructure of these states in terms of quarkonia and tetraquark components as well as scalar glueball content is explored, and preliminary study of the strong and radiative decays of the 0⁺⁺ mesons, together with the scalar glueball masses and partial widths, is also presented.For each state studied in Chapter 3 and 4, a careful analysis together with the numerical estimates is given. As a consequence, we get a reasonable description of these exotic hadrons. In viewing that they are controversial in the present PDG (Particle Data Group) listing, the predicted results may be helpful for upcoming experimental study. Finally, discussions and outlook are briefly given in Chapter 5. To ease the reading, three appendices are provided to clarity the notations and formalisms frequently used and also to elucidate different intermediate steps. Appendix A outlines the basic features and pictures of different hadron structure models in a unified way. Appendix B concerns the group technical details that deal with the Left/Right algebra and D-matrix calculations, which are mainly developed for the calculations occur in Chapter 3. Appendix C is devoted to the concept in the diquark model from the group theoretical point of view. It is partly related to the material in Chapter 3 and 4. Estimates are also made for the flavor wavefunctions and magnetic moments of pentaquark states.