Study Some Properties Of Hadrons In Constituent Quark Model

Baryons are the basic strong interacting systems. The study of hadron spectrum leads to the emergence of quark model and the establishment of quantum chromodynam-ics (QCD), the fundamental theory of strong interaction, just like the study of atomic spectrum leading to the establishment of quantum mechanics. In high energy region, QCD has a characteristic of asymptotic freedom, the perturbation theory can be applied, the calculated results have been tested precisely by experiments. In low energy region, the perturbation theory fails to work due to the characteristic of QCD, color confinement. So to resolve the problem in low energy region from the first principle is very difficult because of the complicate of QCD. The study of hadron spectrum belongs to the low en-ergy region of QCD. Now various non-perturbation methods lattice QCD, QCD sum rule, Dyson-Schwinger equation, phenomenological quark model and so on, are developed. Al-though the methods based on QCD have made a lot of progresses recently, the description of hadron is still far from satisfactory, e.g., failing to describe the excited states of hadrons. QCD has another characteristic, chiral symmetry spontaneous breaking, because of it, the current quark obtains the dynamical mass and changes into constituent quark. The in-teraction between constituent quarks are introduced by gluon exchange and Goldstone boson exchange. In this way, the "QCD inspired" quark models are constructed. Quark models have achieved a great success in describing hadron spectrum and hadron-hadron interaction. It is am powerful tool in the study of hadron physics. In thesis, we study the properties of hadrons in the framework of quark model.The argument of the importance of relativistic effects of hadron in quark model per-sists with the development of quark model. Generally, the relativistic effects should be small enough to be neglected for heavy quark system, whereas the effects may not be able to be neglected for light quark system. However, the calculation of non-relativistic quark model shows that it can describe all the hadrons, from light to heavy, well, although the contribution of kinetic energy to the mass of light hadron exceeds the contribution from the rest masses. In this thesis, the constituent quark models with the commonly used po-tentials, quadratic confinement or linear confinement plus one-gluon exchange potential are employed to study bb system with relativistic (Dirac equation) and non-relativistic (Schrodinger equation) dynamics. The calculations of spectrum, electromagnetic decay widths, electromagnetic transition and hadronic decay widths show that two types of dynamics and two types of confinements can describe the heavy quarkonium. The dif-ferences between two dynamics are small, although the description is a little better in Dirac dynamics in the whole. The relativistic effects can be neglected safely for heavy quarkonium. The behavior of linear confinement is also a little better than the quadratic confinement for the highly excited states.In recent years, a lot of new hadrons are reported by BES, BaBar, Belle and other ex-perimental collaborations, e.g., X(3872),X(3940),Y(4160),Y(4260),Z(4430) and so on. In2005, the BES Collaboration observed a narrow peak in the η’π+π-invariant mass spectrum in the process J/ψ�'η’π+π-and define it as X(1835). BES-Ⅲ confirmed it in the same process. Meanwhile, another two new resonances, X(2120) and X(2370), are also observed in the same process. These states are all pseudoscalar meson with I=0. Much work has been devoted to the underlying structures of these states:charmonia, molecular states, four-quark states, baryonium etc. In this work, the pseudoscalar meson spectrum is determined by the chiral quark model. In the mass calculation, the nonstrange mesons and strange mesons are mixed by the K-meson exchange, the mixing angles are determined by the dynamics of the system. Based on the mass spectra of77and77’, the possible candidates of X(1835), X(2120), X(2370) and η(1760) are assigned. Then the strong decay widths of the states are calculated in the framework of3P0model, and to see the assignment is reasonable or not by comparing with experimental data. The results show that the assignment of η(1760), X(2120),X(2370) to η’(21S0),η’(31S0),η’(41S0) are disfavored in the present model, whereas the assignment of X(1835) to η(41S0) is possi-ble, to confirm the assignment, the branching ratios of X(1835) to πa0(980),πa0(1450) are needed.A promising explanation of some new hadron states is the multiquark states, e.g., N(1440) and A(1405) can be explained as pentaquark states. In the traditional quark model, a baryon consists of three quarks and meson consists of quark-antiquark. In reality, a hadron can have multiquark components. For example, a meson is the com-bination of quark-antiquark, diquark-antidiquark, quark-antiquark-gluon and other Fock components, a baryon is the combination of three-quark, four-quark-antiquark, three- quark-gluon and other Fock components. In order to unify the description of hadrons, the unquenched quark model, in which various Fock components are taken into account, should be constructed. In this thesis, the five-quark components are considered in the study of baryon. The mixing between three-quark component and five-quark components are calculated by using3P0model. The mass shift due to the mixing of five-quark compo-nents are obtained. The results show that mass shifts are rather large, especially for the light baryons. For ground state of baryon, the mass shift can be amended by re-adjusting the model parameters. For excited states of baryon, the effects of mass shift may not be absorbed by the parameter re-adjusting.