We test the validity of the QCD sum rules applied to the meson $Z^+(4430)$, by considering a diquark-antidiquark type of current with $J^{P}=0^{-}$ and with $J^{P}=1^{-}$. We find that, with the studied currents, it is possible to find an acceptable Borel window. In such a Borel window we have simultaneously a good OPE convergence and a pole contribution which is bigger than the continuum contribution. We get $m_Z=(4.52\pm0.09)\GeV$ and $m_Z=(4.84\pm0.14)\GeV$ for the currents with $J^{P}=0^{-}$ and $J^{P}=1^{-}$ respectively. We conclude that the QCD sum rules results favors $J^{P}=0^{-}$ quantum numbers for the $Z^+(4430)$ meson.
After the study of the preclusion of exotic meson states in large-$N_c$ limit QCD, combining Weinberg's opposite proposal, we get different counting orders for a tetraquark operator to create or destroy an one-tetraquark state. Meanwhile, by comparing tetraquark operator with the mesonic and gluonic operators, we find that tetraquark operators are similar with mesonic and gluonic operators in the counting. Furthermore, we find a mixing of different kinds of operators.
Diquarks are found to have the right degrees of freedom to describe the tetraquark poles in hidden-charm to open-charm meson-meson amplitudes. Compact tetraquarks result as intermediate states in non-planar diagrams of the 1/N expansion and the corresponding resonances are narrower than what estimated before. The proximity of tetraquarks to meson-thresholds has an apparent role in this analysis and, in the language of meson molecules, an halving rule in the counting of states is obtained.
It is suggested that the narrow meson state seen in the the SELEX experiment is a $(c\bar{s} s\bar{s})$ tetraquark state. Characteristics of other possible tetraquarks formed out of $c$ and $s$ quarks and antiquarks are considered.
We study the possible tetraquark interpretation of light scalar meson states $a_0(980)$, $f_0(980)$, $κ$, $σ$ within the framework of the non-relativistic potential model. The wave functions of tetraquark states are obtained in a space spanned by multiple Gaussian functions. We find that the mass spectra of the light scalar mesons can be well accommodated in the tetraquark picture if we introduce a three-body quark interaction in the quark model. Using the obtained multiple Gaussian wave functions, the decay constants of tetraquarks are also calculated within the ``fall apart'' mechanism.
Two-ordinary-meson scattering in large-$N_c$ QCD implies consistency criteria for intermediate-tetraquark contributions. Their fulfilment at $N_c$-leading order constrains the nature of the spectrum of genuinely exotic tetraquark states.
We examine the interpretation of the light scalar meson nonet as tetraquark states using QCD sum rules. With the interpolating current for the tetraquark states composed of scalar diquark and scalar antidiquark, first, we construct the QCD sum rules by means of the operator product expansion up to the operators of dimension 8 and show that there is no evidence of the coupling of the tetraquark states to the light scalar meson nonet. In order to have a stable sum rule, we propose a "good" interpolating current for the tetraquarks based on chirality arguments which includes scalar and pseudoscal…
We suggest that the recently discovered charm-strange meson D_sJ(2632), with unusual properties, could be a cyptoexotic tetraquark baryonium state cdd_bars_bar. We predict other four narrow states, as Regge recurrences of D_sJ(2632), below the possible baryon-antibaryon thresholds.
Flavor-exotic tetraquark mesons have recently been observed in the heavy-quark pair sectors of QCD, including two isospin multiplets in the b b-bar sector, Z_b(10610) and Z_b(10650), and one isospin multiplet in the c c-bar sector, Z_c(3900). We identify Z_b and Z_c as tetraquark mesons that are analogs of quarkonium hybrids with the gluon field replaced by an isospin-1 excitation of the light-quark fields. Given the identification of Y(4260) and Z_c(3900) as a ground-state charmonium hybrid and tetraquark, respectively, lattice QCD calculations of the charmonium spectrum can be used to estima…
We embark on systematic explorations of the behaviour of tetraquark mesons (colour-singlet bound states of two quarks and two antiquarks) in the (idealized) limit of a large number $N_c$ of colour degrees of freedom of quantum chromodynamics. Considering the scattering of two ordinary mesons into two ordinary mesons, we start off with formulating a set of selection criteria that should enable us to unambiguously single out precisely those contributions to all encountered scattering amplitudes that potentially will develop tetraquark poles. Assuming that tetraquark mesons do exist and, if so, e…
Considering four-point correlation functions of color-singlet quark bilinears, we investigate, in the large-$N_{\mathrm{c}}$ limit of QCD, the subleading diagrams that involve, in the $s$-channel of meson-meson scattering amplitudes, two-quark--two-antiquark intermediate states. The latter contribute, together with gluon exchanges, to the formation, at the hadronic level, of two-meson and tetraquark intermediate states. It is shown that the two-meson contributions, which are predictable, in general, from leading-order $N_{\mathrm{c}}$-behaviors, consistently satisfy the constraints resulting f…
In the paper we discuss contribution of the doubly intrinsic heavy mechanism into the production of $B_c$ meson, the doubly heavy baryons and the all-charm tetraquark at a future fixed-target experiment at the LHC (AFTER@LHC). The production cross sections and the mean values of Feynman-x for the finale states are presented.
We consider the interplay of the pentaquark states and strange tetraquark states in the decay $Λ^0_b\to K^-J/ψp$. Possible existence of ($cs\bar c\bar u$)-states is taken up and their manifestation in the $K^-J/ψ$-channel is discussed. It is emphasised that these exotic mesons can imitate broad bumps in the in the $pJ/ψ$-channel.
Masses of the ground state light tetraquarks are dynamically calculated in the framework of the relativistic diquark-antidiquark picture. The internal structure of the diquark is taken into account by calculating the form factor of the diquark-gluon interaction in terms of the overlap integral of the diquark wave functions. It is found that scalar mesons with masses below 1 GeV: f_0(600) (σ), K^*_0(800) (κ), f_0(980) and a_0(980) agree well with the light tetraquark interpretation.
The article is devoted to double J/psi-meson production at LHC. Theoretical predictions for pp -> 2J/psi+X cross section at sqrt{s}=7 TeV for different kinematical restrictions are presented. Special attention is payed to possible contribution of novel particles -- tetraquarks build from four c-quarks. According to presented in our article estimates, it is quietly possible to observe at least one of these state (tensor meson) experimentally.
We study the lowest-lying scalar mesons in the QCD sum rule by considering them as tetraquark states. We find that there are five independent currents for each state with a certain flavor structure. By forming linear combinations, we find that some mixed currents give reliable QCD sum rules. Among various tetraquark currents, we consider those which are constructed by the diquarks having anti-symmetric and symmetric flavor structures. That the results of the QCD sum rule derived from the two types of currents are similar suggests that the tetraquark states can have a large mixing between diffe…
In this article, we take the scalar diquarks as point particles and describe them as basic quantum fields, then introduce the SU(3) color gauge interaction and new vacuum condensates to study the nonet scalar mesons as tetraquark states with the QCD sum rules. Comparing with the conventional quark currents, the diquark currents have the outstanding advantage to satisfy the two criteria of the QCD sum rules more easily.
Discussing four-point Green functions of bilinear quark currents in large-$N_c$ QCD, we formulate rigorous criteria for selecting diagrams appropriate for the analysis of potential tetraquark poles. We find that both flavor-exotic and cryptoexotic (i.e., flavor-nonexotic) tetraquarks, if such poles exist, have a width of order $O(1/N_c^2)$, so they are parametrically narrower compared to the ordinary $\bar qq$ mesons, which have a width of order $O(1/N_c)$. Moreover, for flavor-exotic states, the consistency of the large-$N_c$ behavior of "direct" and "recombination" Green functions requires t…
By means of unitarized Chiral Perturbation Theory it is possible to obtain a remarkable description of meson-meson scattering amplitudes up to 1.2 GeV, and generate poles associated to scalar and vector resonances. Since Chiral Perturbation Theory is the QCD low energy effective theory, it is possible then to study its large Nc limit where qqbar states are easily identified. The vectors thus generated follow closely a qqbar behavior, whereas the light scalar poles follow the large Nc behavior expected for a dominant tetraquark or two-meson structure.
The study of two-pseudoscalar and two-photon decays for the scalar meson nonet below 1 GeV is performed within an effective approach in which the scalar resonances are described as (Jaffe's) tetraquark states. The dominant (fall apart decay) and the subdominant (one transverse gluon as intermediate state) decay amplitudes are systematically taken into account. The latter improves the agreement with the experimental data. Possible scenarios concerning the scalar-isoscalar mixing are discussed.
A classification of tetraquark states in terms of the spin-flavour, colour and spatial degrees of freedom has been constructed. The permutational symmetry properties of both the spin-flavour and orbital parts of the quark-quark and antiquark-antiquark subsystems are briefly discussed. This classification is useful both for model-builders and experimentalists. An evaluation of the tetraquark spectrum is obtained from a Iachello mass formula for tetraquark systems. The ground state tetraquark nonet is identified with $f_{0}(600)$, $κ(800)$, $f_{0}(980)$, $a_{0}(980)$. The mass splittings predict…
Scrutinizing the scattering of ordinary mesons in the limiting case of the number of colour degrees of freedom $N_{\rm c}$ of quantum chromodynamics approaching infinity, we formulate Feynman-diagram selection criteria and from these deduce rigorous self-consistency conditions for the manifestation of a tetraquark, a two-quark-two-antiquark bound state, as a pole in the corresponding amplitudes. Our constraints bear rather far-reaching consequences: In particular, all flavour-exotic tetraquarks, composed of four (anti)quarks of disparate flavour, must come in, at least, two variants differing …
Using the QCD sum rule approach we investigate the possible four-quark structure of the recently observed mesons $D_{sJ}^{+}(2317)$, firstly observed by BaBaR, X(3872), firstly observed by BELLE and $D_0^{*0}(2308)$ observed by BELLE. We use diquark-antidiquark currents and work in full QCD, without relying on $1/m_c$ expansion. Our results indicate that a four-quark structure is acceptable for these mesons.
AdS/QCD correspondence predicts the structure of quark-antiquark potential in the static limit. We use this piece of information together with the Salpeter equation (Schrödinger equation with relativistic kinematics) and a short range hyperfine splitting potential to determine quark masses and the quark potential parameters from the meson spectrum. The agreement between theory and experimental data is satisfactory provided one considers only mesons comprising at least one heavy quark. We use the same potential (in the one-gluon-exchange approximation) and these data to estimate constituent diq…
We investigate the widths of the recently observed charmonium like resonances X(3872), Z(4430) and Z_2(4250) using QCD sum rules. Extending previous analyses regarding these states as diquark-antiquark states or molecules of D mesons, we introduce the Breit-Wigner function in the pole term. We find that introducing the width increases the mass at small Borel window region. Using the OPE up to dimension eight, we find that the sum rules based on interpolating current with molecular components give a stable Borel curve from which both the masses and widths of these resonances can be well obtaine…
The relativistic four-quark equations are found in the framework of coupled-channel formalism. The dynamical mixing of the meson-meson states with the four-quark states is considered. The four-quark amplitudes of the tetraquarks with hidden bottom, including u, d, s and bottom quarks, are constructed. The poles of these amplitudes determine the masses and widths of tetraquarks.
We address $q q \bar Q\bar Q$ exotic tetraquark bound states and resonances with a fully unitarized and microscopic quark model. We propose a triple string flip-flop potential, inspired in lattice QCD tetraquarks static potentials and fluxtubes, combining meson-meson and tetraquark potentials. Our potential goes up to the color excited potential, but neglects spin-tensor potentials. To search for bound states and resonances, we first solve the two-body mesonic problem. Then we develop fully unitary techniques to address the four-body tetraquark problem. We fold the four-body Shcrödinger equati…
We look for the existence of stable (by strong interactions) tetraquarks, mesons containing two heavy antiquarks and two light quarks, specifically a $|{\bar b}{\bar b}qq\rangle$ state. This state is viewed as two $|{\bar b}q\rangle $ particles bound by a potential due to the exchange of the four light pseudoscalar and vector mesons, namely the $π,\ η,\ ρ$ and $ω$. Couplings of these particles are obtained by the use of heavy quark symmetry. We find that the $I=0,\ J=1$ state is bound for a large range of parameters, while all other states are not bound (the $I=1,\ J=0$ system may be bound for…
In the framework of the chiral quark model along with complex scaling range, we perform a dynamical study on the low-lying $S$-wave doubly-heavy tetraquark states ($QQ\bar{q}\bar{q}$, $Q=c, b$ and $q=u, d$) with an accurate computing approach, Gaussian expansion method. The meson-meson and diquark-antidiquark configurations within all possible color structures for spin-parity quantum numbers $J^P=0^+$, $1^+$ and $2^+$, and in the $0$ and $1$ isospin sectors are considered. Possible tightly bound and narrow resonance states are obtained for doubly-charm and doubly-bottom tetraquarks with $IJ^P=…
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 relativistic four-quark equations are found in the framework of coupled-channel formalism. The dynamical mixing of the meson-meson states with the four-quark states is considered. The four-quark amplitudes of the tetraquarks, including $u$, $d$, $s$ and bottom quarks, are constructed. The poles of these amplitudes determine the masses and widths of $S$-wave bottom tetraquarks.
The relativistic four-quark equations are found in the framework of coupled-channel formalism. The dynamical mixing of the meson-meson states with the four-quark states is considered. The four-quark amplitudes of the tetraquarks with hidden charm including u, d, s and charmed quarks are constructed. The poles of these amplitudes determine the masses and widths of tetraquarks.
The relativistic four-quark equations are found in the framework of coupled-channel formalism. The dynamical mixing of the meson-meson states with the four-quark states is considered. The approximate solutions of these equations using the method based on the extraction of leading singularities of the amplitudes are obtained. The four-quark amplitudes of cryptoexotic mesons including the quarks of three flavours (u, d, s) and the charmed quark are constructed. The poles of these amplitudes determine the masses of tetraquarks. The mass values of low-lying tetraquarks are calculated.
The relativistic four-quark equations are found in the framework of coupled-channel formalism. The dynamical mixing of the meson-meson states with the four-quark states is considered. The four-quark amplitudes of the negative parity tetraquarks including the quarks of three flavors (u, d, s) and the charmed quark are constructed. The poles of these amplitudes determine the masses of tetraquarks. The mass values of low-lying tetraquarks with the spin-parity JP=0-,1-,2-,3- are calculated.
AdS/QCD correspondence allows to compute the quark-antiquark potential in the static limit. We use this piece of information together with the Salpeter equation (Schroedinger equation with relativistic kinematics) and a short range hyperfine splitting potential to describe the bound states of quark-antiquark. Comparison with experiment permits to fit the parameters of the model, namely the quark masses and the quark potential parameters. A discussion of heavy tetraquark masses, prompted by the hypothesis of a diquark-antidiquark structure for the puzzling state X(3872), is also presented.
We study the light scalar mesons in the QCD sum rule. We construct both the diquark-antidiquark currents $(qq)(\bar q \bar q)$ and the meson-meson currents $(\bar qq)(\bar qq)$. 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 $\bar q q$ 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.
Inspired by the recent observation of exotic resonances $X(4140)$, $X(4274)$, $X(4350)$, $X(4500)$ and $X(4700)$ reported by several experiment collaborations, we investigated the four-quark system $cs\bar c\bar s$ with quantum numbers $J^{PC}=1^{++}$ and $0^{++}$ in the framework of the chiral quark model. Two configurations, diquark-antidiquark and meson-meson, with all possible color structures are considered. The results show that no molecular state can be formed, but the resonance may exist if the color structure of meson-meson configuration is $8\otimes8$. In the present calculation, 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.
Multiquark correlations inside hadrons can have a significant and in some cases even striking impact on the hadron spectrum. We show how such correlations in general, and mesons with a dominant tetraquark content in particular, emerge holographically in the AdS/QCD framework. On this basis, we arrive at a holographic realization of an exceptionally strong four-quark binding and a correspondingly large tetraquark component in the lightest scalar mesons. Higher-lying tetraquark excitations, on the other hand, become too broad to form supernumeral scalar states.
We calculate the next-to-leading order QCD corrections to the perturbative term in the operator product expansion of the spectral functions of light tetraquark currents. By using also configuration space methods we keep the momentum space four-loop calculation to a manageable level. We find that the next-to-leading order corrections to the perturbative term are large and can amount to $O(100\%)$. The corrections to the corresponding Borel sum rules, however, are small since the nonperturbative condensate contributions dominate the Borel sum rules.
In the X(3872) decay, both of the J/ψππ and J/ψπππ branching fractions are observed experimentally, and their sizes are comparable to each other. In order to clarify the mechanism to cause such a large isospin violation, we investigate X(3872) employing a model of coupled-channel two-meson scattering with a ccbar core. The two-meson states consist of D0Dbar*0, D+Dbar*-, J/ψρ, and J/ψω. The effects of the ρ and ω meson width are also taken into account. We calculate the transfer strength from the ccbar core to the final two-meson states. It is found that very narrow J/ψρ and J/ψω peaks appear v…
Considering quantum chromodynamics in the consistent limit of simultaneous correlated increase of the number of colours, $N_{\rm c}$, beyond bounds and decrease of the strong coupling to zero allows for solid statements about qualitative features of the class of "truly exotic" tetraquark mesons carrying four mutually distinct quark flavours. Consistency criteria extracted from correlation functions for two-ordinary-meson scattering suggest the existence of more than one such tetraquarks, at least, of two tetraquarks of identical quark-flavour content and large-$N_{\rm c}$ behaviour of the tota…
In this article, we take the point of view that the charmed scalar meson $D_{s0}(2317)$ be a tetraquark state and devote to calculate its mass within the framework of the QCD sum rules approach in the heavy quark limit. The numerical values for the mass of the $D_{s0}(2317)$ are consistent with the experimental data, there must be some tetraquark component in the scalar meson $D_{s0}(2317)$. Detailed discussions about the threshold parameter and Borel parameter for the multiquark states are also presented.
In this work, we continue to study the mass spectra of fully-heavy tetraquarks $cc\bar{c}\bar{c}$ with the quantum numbers of $J^{PC}=0^{++}, 1^{+-}, 2^{++}$ in the nonrelativistic chiral quark model. With the help of the Gaussian Expansion Method, we present dynamical computations for $cc\bar{c}\bar{c}$ state with considering two structures, meson-meson [$\bar{c}c$][$\bar{c}c$] and diquark-antidiquark [$cc$][$\bar{c}\bar{c}$] and their mixing. The results manifest that the energies of the low-lying states are all higher than the meson-meson thresholds [$\bar{c}c$][$\bar{c}c$]. However, resona…
We compare two frequently discussed competing structures for a stable $\bar b \bar b u d$ tetraquark with quantum numbers $I(J^P) = 0(1^+)$ by considering a meson-meson as well as a diquark-antidiquark creation operator. We treat the heavy antiquarks as static with fixed positions and find diquark-antidiquark dominance for $\bar b \bar b$ separations $r < 0.2 \, \text{fm}$, while for $r > 0.5 \, \text{fm}$ the system essentially corresponds to a pair of $B$ mesons. For the meson-meson to diquark-antidiquark ratio of the tetraquark we obtain around $58\%/42\%$.
The decays of a light scalar meson into a vector meson and a photon ($S\to Vγ)$ are evaluated in the tetraquark and quarkonium assignments of the scalar states. A link with the radiative decays $ϕ\to Sγ$ is established: experimental results for $S\to Vγ$ will allow to understand if the direct, quark-loop contribution to $ϕ\to Sγ$ or the kaon-loop contribution is dominant. Also strong decays $S\to PP$ -where $P$ denotes a pseudoscalar meson- are investigated: the tetraquark assignment works better than the quarkonium one. It is then also discussed why the tetraquark assignment is favoured with …
Scalar mesons are a key expression of the strong coupling regime of QCD. How those with $I_3=0$ couple to two photons supplies important information about which is transiently ${\bar q}q$ or ${\bar {qq}} qq$ or multi-meson molecule or largely glue, provided (i) we know the location of the corresponding poles in the complex energy plane, and (ii) we have reliable predictions from strong coupling QCD of the radiative widths for these compositions. Number (i) is being supplied by careful analyses of high statistics data particularly that coming from $B$, $D$, $J/ψ$ and $ϕ$ decays. Number (ii) is …
With a view to exploring a new kind of phase transition in the process of hadronization of quark-gluon plasma (QGP) we investigate the occurrence of pentaquark baryons and tetraquark mesons in the system. For this purpose, the frame work of an analoguous Saha's ionization formula for the colored ions in the system is used. The study of color-ionic-fraction (CIF) of multiply (color) ionized to unionized quark clusters (termed as "quarkons") as a function of temperature is carried out. It is pointed out that not only the temperature of the fire-ball in the relativistic heavy ion collisions evolv…
Following Weinberg's argument that narrow tetraquark mesons are not precluded in large N_c QCD, we explore the flow of N_c factors needed for the consistency of this picture, and show that they must arise in a novel way, not simply through the usual combinatoric counting.
In recent years, the existence of a hadronically stable $\bar{b} \bar{b} u d$ tetraquark with quantum numbers $I(J^P) = 0(1^+)$ was confirmed by first principles lattice QCD computations. In this work we use lattice QCD to compare two frequently discussed competing structures for this tetraquark by considering meson-meson as well as diquark-antidiquark creation operators. We use the static-light approximation, where the two $\bar{b}$ quarks are assumed to be infinitely heavy with frozen positions, while the light $u$ and $d$ quarks are fully relativistic. By minimizing effective energies and b…
The present work reviews the various aspects of the extension of the large-$N_c$ approach, as had been proposed and developed by 't Hooft, to the case of tetraquark states, which are a category of the general class of exotic states, also called multiquark states, whose internal valence-quark structure does not match with that of ordinary hadrons, and which have received, in recent years, many experimental confirmations. The primary question of describing, or probing, on theoretical grounds, multiquark states is first examined. The signature of such states inside Feynman diagrams in relation wi…
The quarks and gluons are confined objects, they cannot be put on the mass-shell, it is questionable to apply the Landau equation to study the Feynman diagrams in the QCD sum rules. Furthermore, we carry out the operator product expansion in the deep Euclidean region $p^2\to -\infty$, where the Landau singularities cannot exist. The Landau equation servers as a kinematical equation in the momentum space, and is independent on the factorizable and nonfactorizable properties of the Feynman diagrams in the color space. The meson-meson scattering state and tetraquark molecular state both have four…
We present a numerical solution of the four-quark Bethe-Salpeter equation for ground-state scalar tetraquarks with J^PC = 0^++. We find that the four-body equation dynamically generates pseudoscalar-meson poles in the Bethe-Salpeter amplitude. The resulting tetraquarks are genuine four-quark states that are dominated by pseudoscalar meson-meson correlations. Diquark-antidiquark contributions are subleading because of their larger mass scale. In the light quark sector, the sensitivity of the tetraquark wave function to the pion poles leads to an isoscalar tetraquark mass M_sigma ~ 350 MeV which…
We perform a SU(3) analysis for both semi-leptonic and non-leptonic heavy meson weak decays into a pseudoscalar meson and a fully-light tetraquark in 10 or 27 representation. A reduction of the SU(3) representation tensor for the fully-light tetraquarks is produced and all the flavor components for each representation tensor are listed. The decay channels we analysis include $B/D \to U/T~P~lν$, $B/D \to U/T~P $ and $B_c \to U/T~P/D$, with $U/T$ represents a fully-light tetraquark in 10 or 27 representation and $P$ is a pseudoscalar meson. Finally, among these results we list all the golden dec…
We study the light scalar mesons a_0(980) and kappa using N_f = 2+1+1 flavor lattice QCD. In order to probe the internal structure of these scalar mesons, and in particular to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. The inclusion of disconnected diagrams, which are technically rather challenging, but which would allow us to extend our work to e.g. the f_0(980) meson, is introduced and discussed.
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…
