Research On Baryon Resonances In Unquenched Quark Model

QCD is the basic theory that describes strong interaction between hadrons. In high energy sector, a number of problems have been solved with QCD in that small running coupling constant from asymptotic freedom validates perturbative expansion calculation; while in low energy sector, perturbation theory fails and physicists have developed a series of phenomenological model methods to carry out non-perturbative calculation. Considering the effect of chiral symmetry spontaneous breaking in low energy QCD, current quarks acquire constituent masses and interact with each other by exchanging gluons or mesons, in this way constituent quark model has been built, extensively applied to the calculation of hadron spectrum as well as that of interaction between hadron-hadron, and successfully employed to explain a lot of experiment phenomena. It is a powerful tool in the study of hadronic physics.Introduced in the 60s of last century, three-quark model of baryons once perfectly described octet and decuplet baryons. In the meanwhile, theoretical predictions and experimental searches of five-quark states have never faltered since QCD theory doesn’t exclude the existence of baryons in multiquark states. After the negation of ever promising candidate of five-quark states, namely O(1540), strange quark-anti strange quark pairs fluctuation newly found in protons quickly became a hot research topic, and an increasing number of new exotic hadronic states arised as more and more high energy experiment data achieved. Phenomena like these that can’t be explained through classic quark model continue physicists’interests in multiquark states and multiquark components in baryons.Given these factors, it is obliged to modify the classic three-quark model of baryons and construct unquenched quark model including five-quark components to describe properties of baryons. In this thesis, we have done some retry and improvement work based on that of pioneer contributors. As for the dilemma of ordering of the lowest negative-parity N(1535) and the lowest positive-parity N(1440) in low-lying nucleon resonance spectrum, we consider significant qqqqq admixtures into them in the framework of constituent quark model. Specifically, we employ simple version of harmonic oscillator model to describe five-quark system, reduce calculation of energy level by viewing five quarks as light quarks(u, d) of the same mass, and add 8m to compensate the mass difference if containing strange quarks. In terms of hyperfine interaction between quarks, color-spin interaction derived from one gluon exchange potential is applied. In the final step, we adjust model parameters by matching computed five-quark system spectrum with empirical baryon resonance spectrum, and select possible five-quark components mixing with corresponding low-lying nucleon-and A resonances according to the principle that calls for same quantum numbers and approximate masses. The results of our study show that the hyperfine interaction remarkably bring the lowest L=1, namely the lowest positive-parity states down to or below the lowest state with L=0, namely the lowest negative-parity state, and when considering admixtures of qqqqq components into qqq states, the dilemma of ordering in low-lying nucleon resonance spectrum will be well solved. On the other hand, to expand research on light baryons in higher-excited states and consider the contribution of five-quark components, we have constructed the spatial wavefunctions of four-identical-quark systems classified under the rotational group SO(3) and the permutation group S4 and made it possible to construct those of light five-quark system, and laid the foundation for the research on the influence of five-quark components on light baryons in higher-excited states.