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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]

We analyze the accuracy of the pion elastic form factor predicted by a localduality (LD) version of dispersive sum rules. To probe the precision of this theoretical approach, we adopt potential models with interactions that involve both Coulomb and confining terms. In this case, the exact form factor may be obtained from the solution of the Schrödinger equation and confronted with the LD sum rule results. We use parameter values appropriate for hadron physics and observe that, independently of the details of the confining interaction, the deviation of the LD form factor from the exact form factor culminates in the region Q2 " 4-6 GeV2. For larger Q2, the accuracy of the LD description increases rather fast with Q2. A similar picture is expected for QCD. For the pion form factor, existing data suggest that the LD limit may be reached already at the relatively low values Q2 = 4-10 GeV2. Thus, large deviations of the pion form factor from the behavior predicted by LDQCDsum rules for higher values of Q2, as found by some recent analyses, appear to us quite improbable. New accurate data on the pion form factor at Q2 = 4-10 GeV2 expected soon from JLab will have important implications for the behavior of the pion form factor in a broad Q2 range up to asymptotically large values of Q2. [ABSTRACT FROM AUTHOR]

Abstract: We study the pion form factor in a broad range of spacelike momentum transfers within the local-duality version of QCD sum rules. We make use of the recently calculated two-loop double spectral density of the correlator including and terms, which allows us to give predictions for the pion form factor and to study the interplay between the nonperturbative and perturbative contributions to the pion form factor without any reference to the pion distribution amplitude. Our results demonstrate the dominance of the nonperturbative contribution to the form factor up to relatively large values of the momentum transfer: namely, the nonperturbative term, which provides the power correction, gives more than half of the pion form factor in the region . [Copyright &y& Elsevier]