QCD Sum Rules, Instanton And New Hadrons

The inner structure of hadrons is one of the fundamental problems in particle physics. Within the Naive Quark Model, hadrons are composed of qq (for meson) or qqq (for baryon). Such a simple scenario, however, is greatly challenged by the experimental evidence. The growing number of unclassified hadrons, and the discovery of mesons with quantum number not allowed by the Naive Quark Model, both suggest the existence of New Hadrons which are beyond the Naive Quark Model.Glueball, multi-quark states and hybrid are three possible New Hadron structures. They are gluons, multi-quarks and gluon-quarks bound states re-spectively. In this thesis, we discuss the properties of these New Hadrons.This thesis can be divided into two parts. First we review our tools which will be used in our research. After a detailed introduction to the QCD sum rules approach in Chapter 2, we review the basic concepts of instanton physics. As we adopt the QCD sum rules as our main theoretical framework, we only focus on the Single Instanton Apporach which can be used in the framework of QCD sum rules easily.The second part of this thesis includes our researches on some specific New Hadrons.First, in Chapter 4, we consider the direct instanton effect in the QCD sum rules for the 0++ glueball mass. Our result shows that after considering the direct instanton effect, the mass of glueball is greatly reduced. After that we discuss the decay of a scaler glueball. We find that because of non-perturbative effects, the SU(3)f symmetry is respected in the decay of a scalar glueball. We also consider the ratio of the decay widths for the four quark final states decay and two quark final states decay of a scalar glueball. And we predict that compared with scalar qq mesons, the probability of a scalar glueball decaying into multi-hadron final states should be remarkably increased. In Chapter 5 we investigate the multi-quark states. First, we construct two typical molecule four quark states with quantum number 1-+. After consider-ing the direct instanton effects in the QCD sum rules approach, we find that our model can accommodate two different 1-+ four quark mesons near 1.4 GeV. Then we study the decay modes of four-quark states, with diquark structure and molecule structure respectively.We discuss the mass sum rules for 1-+ and 0++ hybrid in Chapter 6. Thanks to the direct instanton effects, we predict a more stable mass prediction for the 0++ hybrid, which reads about 2 GeV.Finally, Chapter 7, is our conclusions and outlooks.