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Aiming at self-consistent descriptions of multiquark hadrons (such as tetraquarks, pentaquarks, hexaquarks) by means of QCD sum rules, we note that the totality of contributions to two-point or three-point correlation functions that involve, respectively, either two or just a single operator capable of interpolating the particular multiquark under study can be straightforwardly disentangled into two disjoint classes defined by unambiguously identifiable members. The first is formed by so-called multiquark-phile contributions which indeed might support multiquarks. In the case of flavour-exotic tetraquarks, by definition composed of four (anti–) quarks of mutually different flavours, a tetraquark-phile contribution has to exhibit two or more gluon exchanges of appropriate topology. The second consists of contributions evidently not bearing any relation to multiquarks; these must be discarded when studying multiquarks by QCD sum rules. The first class only should enter the "multiquark-adequate" QCD sum rules for exotic hadrons. [ABSTRACT FROM AUTHOR]

We propose to increase the factual reliability of descriptions of exotic multiquark hadrons utilizing the approach to bound states of strongly interacting constituents known as QCD sum rules, by allowing exclusively all contributions that potentially bear some relevance for multiquark states to enter the correlation functions that form the main ingredient of this framework. The route to this goal is illustrated for the (presumably least involved) special case of tetraquark states. [ABSTRACT FROM AUTHOR]

We embark on systematic explorations of the behaviour of tetraquark mesons, i.e., colour-singlet bound states of two quarks and two antiquarks, in the (idealized) limit of a large number of colour degrees of freedom, Nc,; of quantum chromodynamics, QCD. 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, emerge in the contributions compatible with our criteria at largest admissible order of Nc; we deduce, for the categories of tetraquarks that exhibit either four or only two different open quark flavours, that the decay rates of these tetraquark types are, at least, of order 1/N²c and that internal consistency requires all the members of the first species to exist pairwise, distinguishable by their favoured two-ordinary-meson decay channels. [ABSTRACT FROM AUTHOR]

We formulate rigorous criteria for selecting diagrams to be taken into account in the analysis of potential polyquark (tetra or penta) poles in QCD. The central point of these criteria is the requirement that the Feynman diagrams for the relevant Green functions contain four- or five-quark intermediate states and the corresponding cuts. It is shown that some diagrams which "visually" seem to contain the four-quark cuts, turn out to be free of these singularities and therefore should not be taken into account when calculating the tetraquark properties. We then consider large-Nc QCD which in many cases provide qualitatively correct picture of hadron properties and discuss in detail the tetraquark states. For the "direct" and the "recombination" four-point Green functions, which may potentially contain the tetraquark poles, we formulate large-Nc consistency conditions which strongly restrict the behaviour of the tetraquark-to-ordinary meson transition amplitudes. In the end, these conditions allow us to obtain constraints on width of the potential tetraquark states at large Nc. We report that both flavor-exotic and cryptoexotic (i.e., flavor-nonexotic) tetraquarks, if the corresponding poles exist, have a width of order O(1/Nc²), i.e. they should be parametrically narrower compared to the ordinary ...q mesons with a width of order O(1/Nc). Moreover, for flavor-exotic states, the large-Nc consistency conditions require two narrow flavor-exotic states, each of these states coupling dominantly to one specific meson-meson channel. [ABSTRACT FROM AUTHOR]

We determine the strong couplings of three mesons that involve, at least, one ηc or J/ψ meson, within the framework of a constituent-quark model by means of relativistic dispersion formulations. For strong couplings of J/ψ mesons to two charmed mesons, our approach leads to predictions roughly twice as large as those arising from QCD sum rules. [ABSTRACT FROM AUTHOR]

Compared with lattice QCD, isospin breaking of heavy-meson decay constants from QCD sum rules agrees for D mesons but disagrees (by a factor of four) for Bmesons. [ABSTRACT FROM AUTHOR]

Revisiting the QCD sum-rule description of exotic hadron states, we argue that, in order to arrive at trustable analyses of, for instance, strong decay widths of multiquarks, it is inevitable to adopt the QCD sum-rule approach beyond the leading perturbative level. [ABSTRACT FROM AUTHOR]

QCD sum rules for decay constants of heavy mesons with u or d quark yield for B mesons much less isospin breaking than lattice QCD but good agreement for D mesons. [ABSTRACT FROM AUTHOR]

We discuss the specific features of extracting properties of the exotic polyquark hadrons (tetraquarks, pentaquarks) compared to the usual hadrons by the QCD sum-rule approach. In the case of the ordinary hadrons, already the one-loop leading-order O(α0 s ) correlation functions provide the bulk of the hadron observables, e.g., of the form factor; inclusion of radiative corrections O(αs) modifies already nonzero one-loop contributions. In the case of an exotic hadron, the situation is qualitatively different: discussing strong decays of an exotic tetraquark meson, which provide the main contribution to its width, we show that the disconnected leading-order diagrams are not related to the tetraquark properties. For a proper description of the tetraquark decay width, it is mandatory to calculate specific radiative corrections which generate the connected diagrams. [ABSTRACT FROM AUTHOR]

Revisiting the strong couplings of three mesons, each of which involves at least one charm quark, proves clear disaccord between quark-model and QCD sum-rule results. [ABSTRACT FROM AUTHOR]

We obtain predictions for B(s) → e+e-γ and B(s) → μ+μ-γ decays. All the contributions containing long-distance QCD effects are calculated in the framework of relativistic quark model. The contributions of the light vector-meson resonances related to the virtual photon emission from valence quarks of the B-meson are included. The highest branching ratios for the radiative leptonic B-decays are B(B0s → e+e-γ) = 18.8 × 10-9 and B(B¯0s → μ+μ-γ) = 12.2 × 10-9. We also give the distribution of the fowardbackward asymmetry. [ABSTRACT FROM AUTHOR]

Within the realm of QCD sum rules, one of the most important areas of application of this nonperturbative approach is the prediction of the decay constants of heavy mesons. However, in spite of the fact that, indisputably, the adopted techniques are, of course, very similar, we encounter rather dissimilar challenges, or obstacles, when extracting from two-point correlators of appropriate heavy-light currents interpolating the mesons, the characteristics of charmed mesons with different spin. In view of this, it seems worthwhile to us to revisit this issue for the case of charmed pseudoscalar mesons D(s) and vector mesons D*(s). [ABSTRACT FROM AUTHOR]

We present the outcomes of a very recent investigation of the decay constants of nonstrange and strange heavy-light beauty vector mesons, with special emphasis on the ratio of any such decay constant to the decay constant of the corresponding pseudoscalar meson, by means of Borel-transformed QCD sum rules. Our results suggest that both these ratios are below unity. [ABSTRACT FROM AUTHOR]

We review the recent progress in the applications of QCD sum rules to hadron properties with the emphasis on the following selected problems: (i) development of new algorithms for the extraction of ground-state parameters from two-point correlators; (ii) form factors at large momentum transfers from three-point vacuum correlation functions: (iii) properties of exotic tetraquark hadrons from correlation functions of four-quark currents. [ABSTRACT FROM AUTHOR]

Within the realm of QCD sum rules, one of the most important areas of application of this nonperturbative approach is the prediction of the decay constants of heavy mesons. However, in spite of the fact that, indisputably, the adopted techniques are, of course, very similar, we encounter rather dissimilar challenges, or obstacles, when extracting from two-point correlators of appropriate heavy-light currents interpolating the mesons, the characteristics of charmed mesons with different spin. In view of this, it seems worthwhile to us to revisit this issue for the case of charmed pseudoscalar mesons D(s) and vector mesons D*(s). [ABSTRACT FROM AUTHOR]

We present the outcomes of a very recent investigation of the decay constants of nonstrange and strange heavy-light beauty vector mesons, with special emphasis on the ratio of any such decay constant to the decay constant of the corresponding pseudoscalar meson, by means of Borel-transformed QCD sum rules. Our results suggest that both these ratios are below unity. [ABSTRACT FROM AUTHOR]

We review the recent progress in the applications of QCD sum rules to hadron properties with the emphasis on the following selected problems: (i) development of new algorithms for the extraction of ground-state parameters from two-point correlators; (ii) form factors at large momentum transfers from three-point vacuum correlation functions: (iii) properties of exotic tetraquark hadrons from correlation functions of four-quark currents. [ABSTRACT FROM AUTHOR]

Our recently completed analysis of the decay constants of both pseudoscalar and vector beauty mesons reveals that in the bottom-quark sector two specific features of the sum-rule predictions show up: (i) For the input value of the bottom-quark mass in the MS scheme mb(mb) ≈ 4.18 GeV, the sum-rule result fB ≈ 210-220 MeV for the B meson decay constant is substantially larger than the recent lattice-QCD finding fB ≈ 190 MeV. Requiring QCD sum rules to reproduce the lattice-QCD value of fB yields a significantly larger b-quark mass: mb(mb) = 4.247 GeV. (ii) Whereas QCD sum-rule predictions for the charmed-meson decay constants fD, ..., ... and ... are practically independent of the choice of renormalization scale, in the beauty sector the results for the decay constants-- and especially for the ratio ...--prove to be very sensitive to the specific scale setting. [ABSTRACT FROM AUTHOR]

In spite of undeniable similarities of the applied techniques, somewhat different challenges are encountered when extracting, from QCD sum rules derived from two-point correlators of appropriate interpolating heavy-light quark currents, the decay constants of charmed mesons of pseudoscalar nature, on the one hand, or of vector nature, on the other hand. This observation justifies a rather careful reassessment of the corresponding results. [ABSTRACT FROM AUTHOR]

We sketch a recent sum-rule extraction of the decay constants of the heavy pseudoscalar mesons D, Ds, B, and Bs from the two-point correlator of heavy-light pseudoscalar currents [1]. Our main emphasis lies on the control over all the uncertainties in the decay constants, related both to the input QCD parameters and to the limited accuracy of the method of sum rules. Gaining this control has become possible by application of our new procedure of extracting hadron observables based on a dual threshold depending on the Borel parameter. For the charmed-meson decay constants, we find fD = (206.2±7.3(OPE)±5.1syst)) MeV, fDs = (245.3±15.7(OPE)±4.5(syst)) MeV. For the beauty mesons, the decay constants turn out to be extremely sensitive to the precise value of the MS mass of the b-quark, mb(mb). By requiring our sum-rule estimate to match the average of the lattice determinations of fB, we extract the rather accurate value mb(mb) = (4.245±0.025) GeV Feeding this parameter value into our sum-rule formalismleads to the beauty-meson decay constants fB = (193.4±12.3(OPE)±4.3(syst)) MeV, fBs = (232.5±18.6(OPE)±2.4syst)) MeV. [ABSTRACT FROM AUTHOR]

Modifying the standard approaches to nonperturbative QCD based on Borel-transformed dispersive sum rules by allowing the effective continuum thresholds required for the implementation of quark-hadron duality to depend on the Borel parameters and on any relevant momentum promises to entail higher accuracy and reliable error estimates for the extracted predictions of hadron features. [ABSTRACT FROM AUTHOR]

We study systematic errors of the ground-state parameters obtained by Shifman–Vainshtein–Zakharov (SVZ) sum rules, making use of the harmonic-oscillator potential model as an example. In this case, one knows the exact solution for the polarization operator, which allows one to obtain both the OPE to any order and the parameters (masses and decay constants) of the bound states. We determine the parameters of the ground state making use of the standard procedures of the method of sum rules, and compare the obtained results with the known exact values. We show that in the situation when the continuum contribution to the polarization operator is not known and is modelled by an effective continuum, the method of sum rules does not allow to control the systematic uncertainties of the extracted ground-state parameters. [ABSTRACT FROM AUTHOR]

We show that, in a model based exclusively on constituent-quark degrees of freedom interacting via a potential, the full axial current is conserved if the spectrum of QQ states contains a massless pseudoscalar. The current conservation emerges nonperturbatively if the model satisfies certain constraints on (i) the axial coupling gA of the constituent quark and (ii) the QQ potential at large distances. We define the chiral point of the constituent quark model as that set of values of the parameters (such as the masses of the constituent quarks and the couplings in the QQ potential) for which the mass of the lowest pseudoscalar QQ bound state vanishes. At the chiral point the main signatures of the spontaneously broken chiral symmetry are shown to be present, namely: the axial current is conserved, the decay constants of the excited pseudoscalar bound states vanish, and the pion decay constant has a nonzero value. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present a sum-rule extraction of decay constants of heavy mesons from the two-point correlator of heavy-light pseudoscalar currents. Our primary concern is to control the uncertainties of the decay constants, induced by both input QCD parameters and limited accuracy of the sum-rule method. Gaining this control is possible by applying our novel procedure for the extraction of hadron observables utilizing Borel-parameter-depending dual thresholds. For the charmed mesons, we obtain fD = (206.2±7.3(OPE)±5.1(syst)) MeV and fDs = (245.3±15.7(OPE)±4.5(syst)) MeV. In the case of the beauty mesons, the decay constants prove to be extremely sensitive to the exact value of the b-quark MS mass mb(mb). By matching our sum-rule prediction for fB to the lattice outcomes, the very accurate b-mass value mb(mb) = (4.245±0.025) GeV is found, which yields fB = (193.4±12.3(OPE)±4.3(syst)) MeV and fBs = (232.5±18.6(OPE)±2.4(syst)) MeV. [ABSTRACT FROM AUTHOR]

We analyse the decay Θs(1/2+) → NK in a non-relativistic Fock space description using three and five constituent quarks for the nucleon and the pentaquark, respectively. Following Jaffe and Wilczek, we assume that quark-quark correlations in spin-zero state play an important role for the pentaquark internal structure. Within this scenario, a strong dynamical suppression of the decay width is shown to be possible only if the pentaquark has an asymmetric “peanut” structure with the strange antiquark in the center and the two extended composite diquarks rotating around. In this case a decay width of ≃ 1 MeV may be a natural possibility. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]