Color Structures Of Multi-quark Systems And Model Study

Quantum chromodynamics (QCD) is generally accepted as the fundamental theory of the strong interaction. High energy processes are calculable using perturbation method due to the asymptotic freedom property of QCD, however, owing to the infrared confinement, low and middle energy strong interaction systems can not be resolved directly according to QCD. Now, effective nonperturbation methods have lattice gauge theory, QCD sum rule and phenomenological quark model. Although lattice gauge theory and QCD sum rule start from QCD first principle, they have some deficiencies. So at present, the QCD-inspired model is still a powerful tool to explore the secret of strong interaction systems.An ordinary hadron as a simplest quark system opens the gate for us to understand QCD, but QCD do not deny the exist of multiquark states. The color structure of a hadron is exclusive, which can help us construct effective quark model but can not provide more information about color structure. In order to obtain more information about color structure, the study on multiquark states is very essential. This thesis employs different quark models to study multiquark states in the framework of phenomenological quark model.First, we propose QCD benzene and a new color structure for tetraquark systems, which enrich color structures for multiquark systems and provide more color structure information for low energy QCD. Basing on similarity between flux tube and chemical bond, we advance QCD isomeric compounds arising from muli-quark states with same quark content but many different color structures. In the framework of string model, we calculate the energy of multiquarks states with different color structures and discuss possible resonance mechanism: color structure resonance, we also analyse the possible effect of QCD benzene in the NN scattering.Second, we study K~-pp state discovered by experiments with quark model, the research is the first time to study K~-pp on the basis of with quark model. In the framework of quark delocalization color screening(QDCSM), K pp forms a symmetrical linear tri-atomic molecule with rms about 1.15 fm. It is a little smaller than one in Ref.[62] due to quarks in protons shrink to the center. The density is about as much as 2.2 times deuteron density. The binding energy is around 80 MeV, it is a little lower than experimental value 115MeV. There is a rather strong attraction in the system, quark delocalization accounts for such strong interaction, the color screening adds a little attraction.Finally, we systematically study tetraquark state uds|-s|- using Gaussian Expansion Method in the framework of QDCSM. we find that tetraquark state uds|-s|- with quantum numbers I = 0 and J~P = 2~+ has binding energy 3MeV and rms 1.5fm. Color screening is responsible for the strong attraction in the system. It has a similar mechanism which binds the deuteron. By this study, we update the concept of quark delocalization and enrich the usage of color screening which avoid the problem occurring in the previous usage. This research combine QDCSM and Gaussian Expansion Method perfectly, which paves the way for accurately solving tetraquark systems in the framework of QDCSM.