Coupled Channel Effects In Heavy Meson Systems

Recently, the Belle and BaBar and other collaborations in B meson factories have reported a number of new hadron states with b or c quarks. The states which aroused extensive interest for theoretical and experimental physicists are the so-called XYZ parti-cles, whose common feature is that they all contain c or c quark in the final states of their strong decay. Theoretically, people have tried to explain these new hadron states from different perspectives. The quantum chromodynamics (QCD) is generally accepted as the fundamental theory of strong interaction, but it is difficult to deal with the lower energy hadronic problems by the first principle of QCD because of the failure of perturbative theory and the complex of nonperturbative theory. However, the QCD-inspired quark models have achieved great success to describe the nucleon-nucleon interaction, spectrum of baryons, heavy quark positronium. When we apply the constituent quark model to ex-plain the new hadron states, although, we can find the states with corresponding quantum number if they are treated as traditional mesons, but the energy of most of states above the threshold of DD mesons is not in good agreement with the experimental date. So people try to explain these particles as four quark states, molecular states, hybrid states and so on except charmonium states, but no one explanation is convincing until now.In the traditional constituent quark model, the meson is consists of two quarks. In fact that the meson system can have four-quark and more quark components besides two-quark component. The contribution from these high Fock state components should not be ignored in meson configuration. These high Fock states can be treated as hadrons which come from the combination of the quark and qnti-quark created from the vacuum with the original valence quark and antiquark. Take charmonium as an example, the light quark pair creation can result in mixing between the bare charmonium state cc and the DD meson pair. Such a coupled channel system will significantly change the meson mass spectrum, and we call it coupled channel effects. Based on constituent quark model and 3P0 model, we calculate mass shifts of some heavy quark mesons which are established well experimentally, and find that the effect on the mass spectrum can not be ignored from four-quark component, which lays the foundation for recalculating the meson mass spectrum in the unquenched quark model. When we establish the quantum state of hadrons, not only the mass spectrum need to be in agreement with the experiment, but also strong decay, leptonic decay and ra-diative decay are important criteria. In this paper we study the properties of the new states X(4160) and X(3915) in terms of mass spectrum and strong decay of charmonium states. According to the mass spectrum of charmonium states predicted by the potential model, the statesλ0(33P0);λ1(33P1);ηc2(21D2),(41S0) all can be candidates for the X(4160). However, only the decay width of the stateηc2(21D2) in our calculation is in good agreement with the experimental date. Therefore, it is reasonable to interpret the charmonium stateηc2(21D2) as the state X(4160). For the state X(3915), although the mass ofχ0(23P0) is compatible with the experimental value, the calculated strong decay width is much larger than experimental data. Hence, the assignment of X(3915) to char-monium stateχ0(23P0) is disfavored in our calculation. X(3915) can be explained as a molecular state D*0D*+ with I(JPC)=0(0++) when the hidden color channel effects are applied.The constituent quark models achieve a great success on hadron physics, there are also several unresolved problems. In order to explain the new hadron states, the development of the quark model is needed. It is a significant attempt to modify the mass spectrum of mesons based on the coupled channel effects and 3P0 model. To identify new hadron states, it is more reasonable to do according to the strong decay width of the states than to the mass spectrum only. This theoretical work is helpful to deepen our understanding of hadron physics and QCD.