Coupled Channel Effects Of Non-strange Quark Spectroscopy Of Positronium

The quantum chromodynamics (QCD) is generally accepted as the fundamental theory of strong interaction among quarks and gluons. This theory have three basic characteristics: asymptotic freedom, confinement in long-range and spontaneous chiral symmetry breaking in low energy region. In high energy region, the theory is calculable using perturbation method. However, because of the infrared confinement and the complexity of QCD, low and intermediate energy strong interaction systems can not be resolved directly from QCD first principle. As a phenomenology nonperturbative method, the QCD-inspired quark mod-els have achieved great success in describing the nucleon-nucleon interaction, spectrum of baryons and meson. In the quark models, meson has a very simple picture:a quark and an anti-quark form bound state with the one-gluon exchange potential in short-range region and confinement in long-range region. Our first work is to calculate the parts of the spectrum of heavy and light mesons. Here a very powerful method for few-body system, gaussian expansion method, is used. The calculated results show that much better meson spectra are obtained. In addition, the method provide a easy to use wavefunctions for the further calculation.In the traditional constituent quark model, the meson is consists of quark and anti-quark. The2-quark system can describe most mesons’properties well. However, there are also several unresolved problems. In fact, the contribution from these high Fock state com-ponents should not be ignored in meson configuration. And QCD does not deny the exist of multiquark states. These high Fock states can be treated as hadrons which come from the combination of the quark and anti-quark created from the vacuum with the original valence quark and antiquark. The high Fock stats will affect the meson spectrum. The effect is also called channel coupling effect. Take charmonium as an example, the quantum effect due to the quark pair creation can be assorted to the interaction between charmonium and double D mesons channel. The coupled channel effect will significantly change the meson mass spectrum. Formerly, our group have calculated the effect on the charmonium from coupled channel effect. Based on constituent quark model and3P0model, the another work is to calculate the mass shifts of some non-strange mesons which are established well experimen-tally, and we find that the coupled channel effect on the non-strange meson mass spectrum can not be ignored.In our calculation, we find that the mass shifts of non-strange meson due to the high Fock states are very large, and the effect can not be absorbed by the model parameters. So, this is a significant attempt. It shows that the quark model should be unquenched, i.e, taking into account of the high Fock components in the study of meson and this work lays the foundation for recalculating the meson mass spectrum in the unquenched quark model.