Quark models for mesons

With the advance of experiment, many high excited states of meson are identified. It is found that many of them are difficult to fit into conventional q$\bar{q}$ meson spectra. Various explanations are proposed. One of them is the four-quark states or molecules of two mesons. In the framework of quark model, by taking into account of multi-body interaction, four-quark states are studied. The results show that the scalar mesons: f0(600), f0(980) and Y(2175) can be well described by four-quark states. Furthermore, unquenched quark model, which incorporates the four-quark components in the conventional q$\bar{q}$ states, is constructed for mesons. By readjusting the model parameters, the model can give a good description of mesons. The model is also used to study the newly discovered XYZ particles