The molecular assignments to the three $P_c$ states and the similar production mechanism between the $Λ_b\to P_c+K$ and $B\to XYZ+K$ convince us the $B$ decaying to a charmonium state plus light mesons could be the appropriate production process to search for the charmoniumlike molecular tetraquarks. In this work, we systematically study the interactions between a charmed (charmed-strange) meson and an anti-charmed (anti-charm-strange) meson, which include the $D^{(*)}\bar{D}^{(*)}$, $\bar{D}^{(*)}\bar{D}_1$, $D^{(*)}\bar{D}_2^*$, $D_s^{(*)}\bar{D}_s^{(*)}$, ${D}_s^{(*)}\bar{D}_{s0}^*$, $D_s^{(*)}\bar{D}_{s1}^{\prime}$, ${D}_s^{(*)}\bar{D}_{s1}$, $D_s^{(*)}\bar{D}_{s2}^*$ systems. After adopting the one-boson-exchange effective potentials, our numerical results indicate that, on one hand, there can exist a serial of isoscalar charmoniumlike $\mathcal{D}\bar{\mathcal{D}}$ and $\mathcal{D}_s\bar{\mathcal{D}}_s$ molecular states, on the other hand, we can fully exclude the charged charmoniumlike states as the isovector charmoniumlike molecules. Meanwhile, we discuss the two-body hidden-charm decay channels for the obtained $\mathcal{D}\bar{\mathcal{D}}$ and $\mathcal{D}_s\bar{\mathcal{D}}_s$ molecules, especially the $D^{*}\bar{D}^{*}$ molecular tetraquarks. By analyzing the experimental data collected from the $B\to XYZ+K$ and the mass spectrum and two-body hidden-charm decay channels for the obtained $\mathcal{D}\bar{\mathcal{D}}$ and $\mathcal{D}_s\bar{\mathcal{D}}_s$ molecules, we find several possible hints of the existence of the charmoniumlike molecular tetraquarks, i.e., a peculiar characteristic mass spectrum of the isoscalar $D^*\bar{D}^*$ molecular systems can be applied to identify the charmoniumlike molecule. We look forward to the future experiments like the LHCb, Belle II, and BESIII Collaborations can test our results with more precise experimental data.
The lowest-lying charmonium-like tetraquarks $c\bar{c}q\bar{q}$ $(q=u,\,d)$ and $c\bar{c}s\bar{s}$, with spin-parity $J^P=0^+$, $1^+$ and $2^+$, and isospin $I=0$ and $1$, are systematically investigated within the theoretical framework of complex-scaling range for a chiral quark model that has already been successfully applied in former studies of various tetra- and penta-quark systems. A four-body $S$-wave configuration which includes meson-meson, diquark-antidiquark and K-type arrangements of quarks, along with all possible color wave functions, is comprehensively considered. Several narrow…
The past decade witnessed the observation of several exotic charmonium-like resonances, some of which are manifestly non-qqbar mesons. We review one of the most popular interpretations of such states: the compact tetraquark in the constituent diquark-antidiquark picture. Moreover, we discuss some unexplored decay channels which are particularly sensitive to the different phenomenological models and could therefore shed some light on the nature of these fascinating states. Some brief comments on the recently observed pentaquarks are also included.
In this talk, we present the recent study on the charmoniumlike states from the e^+e^- colliders, including BESIII, Belle, BaBar, and CLEOc. The talk covers the X(3872) from the Y(4260) radiative transition, the Y states from the initial-state-radiation processes and from e+e- to pi+pi-h_c, and the charged Z_c states.
In the past decade, more and more charmoniumlike and bottomoniumlike states have been reported in experiments, which have led us to extensive discussions on the underlying structure of these states. In this review paper, we briefly summarize the experimental and theoretical status of these observed states.
Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains a charmed quark and antiquark pair but has properties that are incompatible with a conventional charmonium state. While operating at center-of-mass energies from 2 to 4.9 GeV, the BESIII experiment can access a wide mass range of charmonium and charmoniumlike states, and has contributed significantly in this field. We review BESIII results involving conventional charmonium states, including the first observation of M1 transition $ψ(3686) \to γη_c(2S)$ and the…
Searches for tetraquarks and mesonic molecules in lattice QCD are briefly reviewed. In the light quark sector the most serious candidates are the lightest scalar resonances sigma, kappa, a0 and f0. In the hidden-charm sector I discuss lattice simulations of X(3872), Y(4260), Y(4140) and Z^+(4430). The most serious challenge in all these lattice studies is the presence of scattering states in addition to possible tetraquark/molecular states. The available methods for distinguishing both are reviewed and the main conclusions of the simulations are presented.
Many of the XYZ mesons discovered in the last decade can be identified as bound states of a heavy quark and antiquark in Born-Oppenheimer (B-O) potentials defined by the energy of gluon and light-quark fields in the presence of static color sources. The mesons include quarkonium hybrids, which are bound states in excited flavor-singlet B-O potentials, and quarkonium tetraquarks, which are bound states in flavor-nonsinglet B-O potentials. The deepest hybrid potentials are known from lattice QCD calculations. The deepest tetraquark potentials can be inferred from lattice QCD calculations of stat…
Many of the $XYZ$ resonances observed by the Belle, Babar, CLEO and BESIII collaborations in the past decade are difficult to interpret as conventional quark-antiquark mesons, motivating the consideration of scenarios such as multi-quark states, meson molecules, and hybrids. After a brief introduction to QCD sum-rule methods, we provide a brief but comprehensive review of the mass spectra of the quarkonium-like tetraquark states $qQ\bar q\bar Q$, doubly charmed/bottomed tetraquark states $QQ\bar q\bar q$ and the heavy quarkonium hybrid states $\bar QGQ$ in the QCD sum rules approach. Possible …
Different from the usual tetraquark assignment to charged $Z_c(3885)$ and $Z_c(4025)$ charmoniumlike structures, in this letter we propose a universal non-resonant explanation to decode these $Z_c$'s, which is based on a special dynamical behavior of $e^+e^-\to D^{(*)}\bar{D}^*π$. Our study shows that $Z_c(3885)$ and $Z_c(4025)$ are only the reflection from the $P$-wave charmed meson $D_1(2420)$ involved in $e^+e^-\to D^{(*)}\bar{D}^*π$. Obviously, the present work provides a unique perspective, which can be examined by future experiments like BESIII and BelleII.
Four exotic charmonium-like states, i.e. $X(4140)$, $X(4274)$, $X(4500)$, and $X(4700)$, have been observed very recently by LHCb Collaboration in the decay process $B^+\to J/ψϕK^+$ using the 3${\rm fb}^{-1}$ data of $p\bar p$ collision at $\sqrt s= 7$ and $8$ TeV. In this paper, we investigate systematically the properties of hidden charm tetraquark states. The hidden charm tetraquarks form an octet configuration and a singlet configuration according to flavor $SU(3)$ symmetry. Based on a diquark-antidiquark model, the hidden charm tetraquarks spectra are given. The previous XYZ exotic states…
The masses of tetraquark states of all $qc\bar q \bar c$ and $cc\bar c \bar c$ quark configurations are evaluated in a constituent quark model, where the Cornell-like potential and one-gluon exchange spin-spin coupling are employed. All model parameters are predetermined by comparing the theoretical and experimental masses of light, charmed and bottom mesons. The theoretical predictions of the charmoniumlike tetraquarks are compared with the observed $XYZ$ states, and one tentative assignment is suggested. The work suggests that the $X(6900)$ observed by LHCb is likely to be the first radial e…
Charmonium, the spectroscopy of c\bar{c} mesons, has recently enjoyed a renaissance with the discovery of several missing states and a number of unexpected charmonium-like resonances. The discovery of these new states has been made possible by the extremely large data samples made available by the B-factories at the Stanford Linear Accelerator Center and at KEK in Japan, and at the CESR e^+e^- collider at Cornell. Conventional c\bar{c} states are well described by quark potential models. However, many of these newly discovered charmonium-like mesons do not seem to fit into the conventional c\b…
We present a dynamical approach for description of the multi-quark states that is based on an effective interaction Lagrangian describing the coupling of hadrons to their constituent quarks. First, we explore the consequences of treating the $X(3872)$ meson as a tetraquark bound state. We calculate the decay widths of the observed channels and conclude that for reasonable values of the size parameter of the $X(3872)$ one finds consistency with the available experimental data. Then we have critically checked the tetraquark picture for the $Z_c(3900)$ state by analyzing its strong decays. We fou…
We consider $X(3872)$ and $Y(4140)$ as the vector tetraquark states of $X_c^0\equiv c\bar c u\bar u(d\bar d)$ and $c\bar c s\bar s$, respectively. By connecting $Λ_b\to X_c^0Λ$ to $B^-\to X_c^0 K^-$, we predict that the branching ratios of $Λ_b\to Λ(X(3872)^0\to) J/ψπ^+π^-$ and $Λ_b\to Λ(Y(4140)\to) J/ψϕ$ are $(5.2\pm 1.8)\times 10^{-6}$ and $(4.7\pm 2.6)\times 10^{-6}$, which are accessible to the experiments at the LHCb, respectively. The measurements of these $Λ_b$ modes would be the first experimental evidences for the $XYZ$ states in baryonic decays.
Many of the XYZ mesons discovered in the last decade can be identified as bound states in Born-Oppenheimer (B-O) potentials for a heavy quark and antiquark. They include quarkonium hybrids, which are bound states in excited flavor-singlet B-O potentials, and quarkonium tetraquarks, which are bound states in flavor-nonsinglet B-O potentials. We present simple parameterizations of the deepest flavor-singlet B-O potentials. We infer the deepest flavor-nonsinglet B-O potentials from lattice QCD calculations of static adjoint mesons. Selection rules for hadronic transitions are used to identify XYZ…
The XYZ mesons are unexpected mesons containing a heavy quark-antiquark pair that have been discovered during the last decade. The models for the XYZ mesons that have been proposed include conventional quarkonium, quarkonium hybrids, and quarkonium tetraquarks, whose four constituents can be clustered in several possible ways. None of the models have provided a compelling pattern for the XYZ mesons. The most promising theoretical approaches within QCD are lattice QCD for the c c-bar mesons, lattice NRQCD for the b b-bar mesons, and the Born-Oppenheimer approximation. The additional hints that …
The XYZ mesons are mesons that contain a heavy quark and antiquark but have properties that seem to require additional constituents. Some of them are electrically charged, so they must be tetraquark mesons whose additional constituents are a light quark and antiquark. The list of XYZ mesons has grown to about two dozen over the last decade. A promising approach to understanding these mesons within QCD is the Born-Oppenheimer approximation, which reduces the problem to the solution of the Schrodinger equation in potentials that can be calculated using lattice QCD. The Born-Oppenheimer approxima…
Understanding the nature of the exotic XYZ resonances is one of the open problems in hadronic spectroscopy. Despite the experimental efforts, the structure of these particles still lacks of an accepted theoretical framework. We propose to use the $Z_c^{(\prime)} \to η_c ρ$ decays as a possible discriminant between two of the most popular models: the compact tetraquark and the loosely bound meson molecule. We show that the predictions obtained within the two pictures are significantly different and therefore the proposed decay channel might shed some light on the nature of these states.
The past seventeen years have witnessed tremendous progress on the experimental and theoretical explorations of the multiquark states. The hidden-charm and hidden-bottom multiquark systems were reviewed extensively in [Phys. Rept. 639 (2016) 1-121]. In this article, we shall update the experimental and theoretical efforts on the hidden heavy flavor multiquark systems in the past three years. Especially the LHCb collaboration not only confirmed the existence of the hidden-charm pentaquarks but also provided strong evidence of the molecular picture. Besides the well-known $XYZ$ and $P_c$ states,…
We demonstrate that the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effect. Especially the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix which are qualified as states. We propose a way to distinguish cusp effects from genuine states and demonstrate that (not all of) the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effects. Especially, we show that the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix, since the normal kinematic cusp effect cannot produc…
Quantum Chromodynamics, the theory of strong interactions, predicts several types of bound states. Among them are mesons ($q\bar{q}$) and baryons ($qqq$), which have been the only states observed in experiments for years. However, in the last decade, many states that do not fit this picture have been observed at $B$-factories (BaBar, Belle and CLEO), at $τ$-charm facilities (CLEO-c, BESIII) and also at proton-proton colliders (CDF, D0, LHCb, ATLAS, CMS). There is growing evidence that at least some of the new charmonium- and bottomonium-like states, the so-called XYZ mesons, are new forms of m…
Diquarks with $J^{P}=0^{\pm}$, $1^{\pm}$ containing a heavy (charm or bottom) quark and a light quark are investigated using QCD Laplace sum rules. Masses are determined using appropriately constructed gauge invariant correlation functions, including for the first time next-to-leading order perturbative contributions. The $J^P=0^+$ and $1^+$ charm-light diquark masses are respectively found to be 1.86$\pm$0.05 GeV and 1.87$\pm$0.10 GeV, while those of the $0^+$ and $1^+$ bottom-light diquarks are both determined to be 5.08$\pm$0.04 GeV. The sum rules derived for heavy-light diquarks with negat…
We discuss the charge conjugation quantum number for tetraquarks or meson-meson molecules, seen as possible interpretations of the newly found $XYZ$ charmonium-like resonances.
Recent results on the hadron spectroscopy from lattice QCD are reviewed with emphasis on the meson sector and in particular on quarkonium-like $XYZ$ states. I report on the first rigorous treatment of the near-threshold states $X(3872)$ and $D_s^0(2317)$, and the lattice searches for $Z_c^+(3900)$, $X(4140)$ and double-charm tetraquark states. Meson resonances in light, strange and charm sector are reviewed, where the resonances masses as well as the strong decay widths are reported. The first lattice QCD simulation of two coupled-channels is discussed.
