Narrow Results

Starting from last decade of this century, many charmonium-like states, also called as XYZ states, have been discovered at different experiments. These states are located at the charmonium energy region, but carry properties that can not be explained as conventional charmonium states. They are considered as good candidates of exotic hadrons. Here, recent experimental activities on XYZ states will be presented, including new decay and production information of X(3872), new measurements of the vector states Y, and new results of the charged Z_c(3900) states from e⁺e⁻ machine and in b-flavor decays.

In this talk, we summarize our recent works on the tetraquark mixing framework for the two light-meson nonets in the J^PC = 0⁺⁺ channel, the light nonet [a₀(980), $K_0^{*}\left(700\right)$, f₀(500), f₀(980)] and the heavy nonet [a₀(1450), $K_0^{*}\left(1430\right)$, f₀(1370), f₀(1500)]. We briefly explain this mixing framework and present various phenomenological signatures to support this picture.

We systematically study the mass spectrum of the S-wave fully-heavy tetraquark states $QQ{\overline{Q}}^{\prime }{\overline{Q}}^{\prime }\left(Q,Q^{\prime }=c,b\right)$ using two nonrelativistic quark models. We include the mixing effect between the $6_c-{\overline{6}}_c$ and ${\overline{3}}_c-3_c$ color configurations and find that the $6_c-{\overline{6}}_c$ component may play a quite important role in the ground state. The obtained mass spectrum show that the ground states are located above the di-meson thresholds and may not support the existence of a bound tetraquark state $QQ{\overline{Q}}^{\prime }{\overline{Q}}^{\prime }$.

The strange pentaquarks with hidden heavy quark pair ($q^{3}c\overline{c}$ and $q^{3}b\overline{b}$) are investigated by the coupled-channel quark cluster model. Two types of the q³ color-octet configurations are found to provide the attraction, which makes bound states, sharp resonances, and cusps in the baryon meson scattering. A resonance appears at around 4500 MeV in the strange hidden charm sector. Such structures are more clearly seen in the hidden bottom systems.

We study the configuration mixing effects on baryons in the large N_c limit. We present the results of a complete analysis of the spectrum of all the states in the N = 2 quark model band with N_f = 3.

Recently, LHCb collaboration has confirmed the state X(4140), with a much larger width $\text{Γ}=83\pm {21}_{-14}^{+21}$ MeV than the previous experimental measurements, which has confused the understanding of its nature. We have investigated the possibility of the χ_c1(3P) interpretation for the X(4140), considering the mass and the strong decay properties.

A written account of my talk delivered at the Harald Fritzsch Memorial Symposium in Munich is given. The emphasis is on Harald’s achievements specifically in quark physics and in quantum chromodynamics. Beyond that exemplary results for low-energy baryons are discussed, as obtained over the past years in the framework of the relativistic constituent-quark model based on Goldstone-boson-exchange dynamics.

As an example for exclusive near threshold meson production, we investigate the production of the pseudo-scalar η meson in a microscopic meson exchange (MECM) and in a relativistic quark model, based on a gluon or instanton pair creation (RQM). We describe the baryons as covariant quark – scalar diquark systems with harmonic confinement; the excitation of intermediate baryon resonances is accounted for by meson or by colorless 2-gluon (Pomeron) exchange in the MECM and RQM, respectively. We find that both models account for the energy dependence of the total cross section near the η threshold, reflecting the dominance of the S₁₁(1535) baryon resonance. As a more stringent test we propose the recoilless excitation of the η meson in nuclei.

We carefully investigate the reliability of the propagator Pole Approximation, i.e. the approximation of retaining only the propagator poles in the evaluation of the Mandelstam covariant expression for the electromagnetic current of the pion. Different frames are analyzed, in order to find the most suitable one for calculating the pion form factor within the proposed approximation. It turns out that the approximation is more accurate in the frame where q⁺ is maximal. The relevance of the Pole Approximation is briefly discussed in view of calculations of hadron form factors based on wave functions generated by dynamical models.

The mixing of scalar mesons is an open problem. In this work, the idea of the mixing of scalar mesons will be applied to dibaryon system on quark level. By introducing the mixing of scalar mesons, we dynamically investigate the structure of possible nonstrange dibaryons in the chiral SU(3) quark model by solving the resonating group method equation. The results show that no matter what kind of mixing is considered, the binding energies of nonstrange dibaryons would become stable if reasonable parameters are used.

The ³P₀ model is used to investigate light-strange mesons strong decays. Decay amplitudes and decay widths are evaluated by using relativistic and non-relativistic simple harmonic oscillator (SHO) wave-functions. Numerical results are given for allowed open-flavor decay modes for and mesons. The results for the relativistic model are in good agreement with the recent published data.