Local-duality QCD sum rules for strong isospin breaking in the decay constants of heavy-light mesons.

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 [Formula: see text] and small light-quark mass [Formula: see text].

Tetraquark and two-meson states at large [Formula: see text].

Considering four-point correlation functions of color-singlet quark bilinears, we investigate, in the large-[Formula: see text] limit of QCD, the subleading diagrams that involve, in the -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 [Formula: see text]-behaviors, consistently satisfy the constraints resulting from the [Formula: see text] expansion procedure and thus provide a firm basis for the extraction of tetraquark properties from [Formula: see text]-subleading diagrams.

OPE, charm-quark mass, and decay constants of D and Ds mesons from QCD sum rules.

We present a sum-rule extraction of the decay constants of the charmed mesons D and Ds from the two-point correlator of pseudoscalar currents. First, we compare the perturbative expansion for the correlator and the decay constant performed in terms of the pole and the running MS¯ masses of the charm quark. The perturbative expansion in terms of the pole mass shows no signs of convergence whereas reorganizing this very expansion in terms of the MS¯ mass leads to a distinct hierarchy of the perturbative expansion.