Narrow Results

The light scalar nonets are studied using the QCD sum rules for the tetraquark operators. The operator product expansion for the correlators is calculated up to dimension 12 and this enables us to perform analyses retaining sufficient pole-dominance. To classify the light scalar nonets, we investigate the dependence on current quark mass and flavor dynamics. Especially, to examine the latter, we study separately SU(3) singlet and octet states, and show that the number of annihilation diagrams is largely responsible for their differences, which is also the case even after the inclusion of the finite quark mass. Our results support the tetraquark picture for isosinglets, while that for octets is not conclusive yet.

We study the light scalar mesons in the QCD sum rule. We construct both the diquark-antidiquark currents and the meson-meson currents . We find that there are five independent currents for both cases, and derive the relations between them. For the meson-meson currents, five independent currents are formed by products of color singlet pairs or color octet pairs. However, they can be related to each other, and the relations are derived. We obtain the masses of the light scalar mesons which are consistent with the experiments.

In this study, the bound state energy of a four-quark system was analytically calculated as a two heavy–heavy anti-quarks $\bar{b}\bar{b}$ and two light–light quarks $ud$. Tetraquark was assumed to be a bound state of two-body system consisting of two mesons, each containing a light quark and a heavy antiquark. Due to the presence of heavy mesons in the tetraquark, Born–Oppenheimer approximation was used to study its bound states. To assess the bounding energy, Schrödinger equation was solved using lattice QCD $\bar{b}\bar{b}$ potential, having expanded the tetraquark potential $\bar{b}\bar{b}ud$ up to 11th term. Binding energy state and wave function, however, were obtained in the scalar $u/d$ channel. Graphical results for wave functions obtained versus antiquark–antiquark distance $\bar{b}\bar{b}$ confirmed the existence of the tetraquark $\bar{b}\bar{b}ud$. Analytical bound state energy obtained here was in good agreement with several numerical ones published in the literature, confirming the accuracy of the approach taken here.

Using large N_c counting rule, it is argued that tetra-quark resonances do not exist. Also it is pointed out that there exists the violation of exchange degeneracy in the exotic KN scattering channel. It implies either the failure of resonance saturation assumption or it suggests the existence of exotic baryon resonances in such a channel.

The interesting state X(1600) with J^PCI^G = 2⁺⁺2⁺ cannot be a conventional meson in the quark model. Using a mixed interpolating current with different color configurations, we investigate the possible existence of X(1600) in the framework of QCD finite energy sum rules. Our results indicate that both the "hidden color" and coupled channel effects may be quite important in the multiquark system. We propose several reactions to look for this state.

Belle and BaBar have recently reported observations of meson states that decay to final states containing a c and -quark but with properties that do not match well to those expected for any of the as-yet unassigned charmonium meson states. Among the suggested interpretations for these states are meson-antimeson molecular states, diquark-antidiquark tetraquarks and/or -gluon hybrid mesona. I briefly review what is experimentally known about these states and comment on the status of recent attempts to classify them.

The cross-sections of $e^{+}{e}^{-}\to h_c{\pi }^{+}{\pi }^{-}$ and ${\chi }_{c0}\omega$ were measured by the BESIII experiment. In both cross-section distributions, there are structures at a mass of about $4220\mathrm{\text{MeV}}∕c^2$. A combined fit is performed to the two cross-section distributions, assuming the structures are due to the same vector resonant state, the $Y(4220)$. The parameters of the $Y(4220)$ are determined using two fit methods. The ratios $\Gamma (Y(4220)\to {\chi }_{c0}\omega )∕\Gamma (Y(4220)\to h_c{\pi }^{+}{\pi }^{-})$ are obtained, which may help in the understanding of the nature of this structure. Although a similar work was done previously, all the multiple solutions in our fits are taken into account and our conclusions are more precise and complete.

We study the spectra of the doubly charmed tetraquark states in a diquark–antidiquark model. The doubly charmed tetraquark states form an antitriplet and sextet configurations according to flavor SU(3) symmetry. For the tetraquark state $[q{q}^{\prime }][\bar{c}\bar{c}]$, we show the mass for both bound and excited states. The two-body decays of tetraquark states $T^{cc}[0^{+}]$ and $T^{cc}[1^{--}]$ to charmed mesons have also been studied. In the end, the doubly charmed tetraquarks decays to a charmed baryon and a light baryon have been studied in the SU(3) flavor symmetry.

We study the sigma meson in the QCD sum rule. We use the linear combination of five independent currents. We find that for some cases the Borel mass stability and threshold value stability are both weak, while for some cases not. To know the reason for this we ought to consider the π-π continuum contribution in the phenomenological side of the QCD sum rule. Then we study the stability of the extracted mass against the Borel mass and the threshold value and find the σ(600) mass at 530 MeV ± 40 MeV.

In the framework of two flavor quark-meson model we study the effect of mixing between effective quarkonium and tetraquark fields on chiral phase transition. Depending on the values of different parameters we explore different scenarios. The common feature among all of them is that the transition for quarkonium and tetraquark happening at the same temperature for all densities. We find a sufficiently strong negative cubic self interaction coupling constant of the tetraquark field can drive the chiral phase transition to first order even at zero quark chemical potential.