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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 from the $$1/N_{\mathrm {c}}$$ expansion procedure and thus provide a firm basis for the extraction of tetraquark properties from $$N_{\mathrm {c}}$$ -subleading diagrams. We find that, in general, tetraquarks, if they exist in compact form, should have narrow decay widths, of the order of $$N_{\mathrm {c}}^{-2}$$ . For the particular case of exotic tetraquarks, involving four different quark flavors, two different types of tetraquark are needed, each having a preferred decay channel, to satisfy the consistency constraints. [ABSTRACT FROM AUTHOR]

We correct Eq. (1) of the original paper, where the term. [ABSTRACT FROM AUTHOR]

We discuss the leptonic decay constants of heavy-light mesons by means of Borel QCD sum rules in the local-duality (LD) limit of infinitely large Borel mass parameter. In this limit, for an appropriate choice of the invariant structures in the QCD correlation functions, all vacuum-condensate contributions vanish and all nonperturbative effects are contained in only one quantity, the effective threshold. We study properties of the LD effective thresholds in the limits of large heavy-quark mass mQ and small light-quark mass mq. In the heavy-quark limit, we clarify the role played by the radiative corrections in the effective threshold for reproducing the pQCD expansion of the decay constants of pseudoscalar and vector mesons. We show that the dependence of the meson decay constants on mq arises predominantly (at the level of 70-80%) from the calculable mq-dependence of the perturbative spectral densities. Making use of the lattice QCD results for the decay constants of nonstrange and strange pseudoscalar and vector heavy mesons, we obtain solid predictions for the decay constants of heavy-light mesons as functions of mq in the range from a few to 100 MeV and evaluate the corresponding strong isospin-breaking effects: fD+-fD0=(0.96±0.09)MeV, fD∗+-fD∗0=(1.18±0.35)MeV, fB0-fB+=(1.01±0.10)MeV, fB∗0-fB∗+=(0.89±0.30)MeV. [ABSTRACT FROM AUTHOR]