Research On Exotic States In Hadron Spectrum

Our present understanding of the strong interactions is that it is described as the non-Abelian gauge field theory Quantum Chromodynamics(QCD) [1,3-6] which describes the interactions of quarks and gluons. Even though in QCD we have a theory of the strong interactions, unfortunately, calculating the properties of hadrons from the QCD lagrangian has proven to be a very difficult task in this strongly coupled nonlinear theory. Although we have made great progress in understanding strong interaction especially in the ultraviolet region, properties of medium and low energy QCD still present challenges to particle physicist and remain to be understood. Until we can both predict the properties of the physical states of the theory and confirm these predictions by experiment we can hardly claim to understand QCD.Because of the difficulties in solving QCD exactly to obtain the properties of the physical states of the theory, we have resorted to various approximation methods. The most promising of these is lattice QCD(redefine the problem on a discrete spacetime lattice) [7-9], Although a great deal of progress has been made, these calculations take enormous amounts of computer time and progress has been slow. A less rigorous approach which has proven to be quite useful and reasonably successful, has been to use phenomenological models of hadron structure to describe hadron properties. These models predict multiquark states, glueballs, and hybrids, in addition to the conventional q(?) mesons and qqq baryons of the quark model. This thesis mainly concentrates on the the hadrons which are recently discovered by the BES Collaboration and the Babar Collaboration, mostly we use phenomenological models of hadron structure to estimate the hadron mass, to study the production properties and decay properties and so on.We have studied X(1835) from both the chiral soliton model and the quark model, we propose X(1835) is a baryonium with quantum number J^PC = 0^-+, 1^G = 0⁺, in this picture, we predict X(1835) have large branch ratio intoη'ππ, which has been verified by experiment. Moreover, the small production rate of X(1835) inγ(1S)→γ(1835) and J/ψ→ωX(1835) can be understood. Both the quantum number and the baryonium structure suggest us to study the q³(?)³ system in order to understand nature of X(1835), as a byproduct both the p(Λ|-) andΛ(Λ|-) enhancements are considered in the same framework. We exactly calculate the spectrum of q³(?)³ in quark model with colormagnetic interaction, we find that the states corresponding to the p(?) enhancement, p(Λ|-) enhancement,Λp(Λ|-) enhancement,...have rather large negative colormagnetic energy, this indicates that these states should be visible in experiments. The relation between the lower energy states and the baryon-antibaryon enhancements needs to be studied further.The BES Collaboration also reported an extremely broad signal X(1576) in the K⁺K^- invariant mass spectrum in the J/ψ→π°K⁺K^- channel. We propose X(1576) is a diquark-antidiquark bound state, then the large width can be naturally understood. In this scheme there exist a vector nonet similar to the famous scalar nonet (α₀(980),f₀(980),κet al), the masses of members of the nonet are estimated, the decay of the nonet is analyzed in detail, it can decay into two pseudoscalars or one pseudoscalar and one vector meson. We predict that X(1576) almost can not decay intoÏ€⁺Ï€^-, and another favored decay channel X(1576)→φπ⁰ is suggested, which are crucial tests of our scheme, can distinguish different models. Furthermore, J/ψdecay channels to search other members of the nonet are suggested.The good 1^-- strangeonium hybrid candidate Y(2175) is studied in detail from both the flux Lube model and the constituent gluon model, which is observed in e⁺e^-→φf₀(980) with initial state radiation. The mass estimates in both models support Y(2175) should be a strangeonium hybrid, and the decay pattern is very similar in both models. Y(2175) mainly decays into K₁(1270)K, K₁(1400)K and K*(1410)K in hybrid scheme, which is supported by the present experimental data. Furthermore 2³D₁ strange quarkonium is investigated in order to confirm whether Y(2175) is a exotic hybrid state or a common q(?) state, and the experimental search of the decay modes KK, K*K*, K(1460)K,h₁ (1380)ηis suggested to distinguish the two pictures, which are favored in 2³ D₁ s(?) picture, however are suppressed in the hybrid scheme.Hadron spectrum is a very difficult problem, and there is no "Theory of Spectrum" in QCD, understanding the hadron structure and hadron properties from first principle is especially demanded, since many new hadrons have been observed in recent years. There arc some important, very difficult problems need to be answered, such as the absence of exotics, we expect great progress in hadron spectrum in future.