| Popis: |
In this review article I describe a model of the nucleon whose ingredients are quark, meson and hybrid or meikton fields. The strategy has been to choose a suitable lagrangian and treat it in the mean field approximation. The model is easily extended to low-lying states of other baryons - strange and nonstrange. The model is chiral invariant with small but appropriate chiral invariance breaking terms. It is based on the color dielectric model of Nielsen and Patkos. The vanishing of the color dielectric function in the vacuum, an indispensable element of the model, has been justified on the basis of the area law of QCD. The model has essential connection to the two major characteristics of the QCD vacuum, viz, gluon and quark condensates. Specifically, fluctuations of the two condensates, hybrids and mesons, are ingredients of the model. Physical quantities of the model have the same large Nc behavior as the corresponding quantities in QCD. The last three features encourage us to hope that the model may have basis in QCD. The question of coexistence of quarks and bosons in an effective lagrangian has been considered with care. The conclusion is that in a confining model the quark field must be retained. But only tree graphs should be evaluated. The contributions of quark loops appear through the bosonic lagrangian. I argue that no special action needs to be taken in incorporating the role of the trace anomaly of QCD in an effective lagrangian. The trace of stress-energy tensor of our effective lagrangian is nonzero at the classical level. It should be interpreted as the manifestation of the QCD trace anomaly. A difficulty which plagues most confining models is that the product of the nucleon mass and the rms matter radius turns out to be ∼ 6 or greater. This problem has been essentially eliminated by the inclusion of the 't Hooft interaction. |
