The hidden-charm pentaquark and tetraquark states

In the past decade many charmonium-like states were observed experimentally. Especially those charged charmonium-like $Z_c$ states and bottomonium-like $Z_b$ states can not be accommodated within the naive quark model. These charged $Z_c$ states are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. Recently, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. We list the puzzles and theoretical challenges of these models when confronted with the experimental data. We also discuss possible future measurements which may distinguish the theoretical schemes on the underlying structures of the hidden-charm multiquark states.Comment: Review accepted by Physics Reports, 152 pages, 66 figures, and 29 table

Isospin symmetry breaking of K and K* mesons

We use the method of QCD sum rules to investigate the isospin symmetry breaking of K and K* mesons. The electromagnetic effect, difference between up and down current-quark masses and difference between up and down quark condensates are important. We perform sum rule analyses of their masses and decay constant differences, which are consistent with experimental values. Our results yield Delta f_K = f_{K^0} - f_{K^\pm} = 1.5 MeV.Comment: 9 pages, 7 figures, one reference adde

The consistency of estimator under fixed design regression model with NQD errors

In this article, basing on NQD samples, we investigate the fixed design nonparametric regression model, where the errors are pairwise NQD random errors, with fixed design points, and an unknown function. Nonparametric weighted estimator will be introduced and its consistency is studied. As special case, the consistency result for weighted kernel estimators of the model is obtained. This extends the earlier work on independent random and dependent random errors to NQD case

Light Scalar Mesons in the QCD Sum Rule

We study the light scalar mesons in the QCD sum rule. We find that there are five independent scalar tetraquark currents in the local form, and we perform QCD sum rule analysis using both these currents and their combinations. Compared with the sum rules by conventional \bar{q} q currents, our result supports a tetraquark structure for low-lying scalar mesons.Comment: 4 pages, 1 figure. Talk given at the Yukawa International Seminar (YKIS) 2006, "New frontiers in QCD", Kyoto, Japan, 20 Nov. - 8 Dec. 200

Mass spectra of Zc and Zb exotic states as hadron molecules

We construct charmonium-like and bottomonium-like molecular interpolating currents with quantum numbers J(PC)=1(+-) in a systematic way, including both color singlet-singlet and color octet-octet structures. Using these interpolating currents, we calculate two-point correlation functions and perform QCD sum rule analyses to obtain mass spectra of the charmonium-like and bottomonium-like molecular states. Masses of the charmonium-like q_bar c c_bar q molecular states for these various currents are extracted in the range 3.85-4.22 GeV, which are in good agreement with observed masses of the Zc resonances. Our numerical results suggest a possible landscape of hadronic molecule interpretations of the newly-observed Zc states. Mass spectra of the bottomonium-like q_bar b b_bar q molecular states are similarly obtained in the range 9.92-10.48 GeV, which support the interpretation of the Zb(10610) meson as a molecular state within theoretical uncertainties. Possible decay channels of these molecular states are also discussed.Comment: 15 pages, 3 figures, 2 tables, accepted by PR

The Possible J^{PC}=0^{--} Exotic State

In order to explore the possible existence of the exotic $0^{--}$ state, we have constructed the tetraquark interpolating operators systematically. As a byproduct, we notice the $0^{+-}$ tetraquark operators without derivatives do not exist. The special Lorentz structure of the $0^{--}$ currents forbids the four-quark correction to the spectral density. Now the gluon condensate is the dominant power correction. Within the framework of the finite energy sum rule, none of the seven interpolating currents supports a resonant signal. Therefore we conclude that the exotic $0^{--}$ state does not exist below 2 GeV, which is consistent with the current experimental observations.Comment: 12 pages, 27 figure

Light vector meson and heavy baryon strong interaction

We calculate the coupling constants between the light vector mesons and heavy baryons within the framework of the light-cone QCD sum rule in the leading order of heavy quark effective theory. Most resulting sum rules are stable with the variations of the Borel parameter and the continuum threshold. The extracted couplings will be useful in the study of the possible heavy baryon molecular states