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We present a study of the strong isospin-breaking (IB) effect, due in QCD to the difference between u - and d -quark masses, in the leptonic decay constants of charmed and beauty pseudoscalar and vector mesons using the method of QCD sum rules. We apply the sum-rule analysis to the decay constants of mesons containing one heavy quark and one light quark with the light mass in the range from the average u / d quark mass to the strange-quark mass. We then analyse the dependence of the decay constants on the light-quark mass and extract with good accuracy the IB ratios of decay constants at leading order in the mass difference ( m d − m u ) , obtaining: ( f D + − f D 0 ) / f D = 0.0047 ( 6 ) , ( f D ⁎ + − f D ⁎ 0 ) / f D ⁎ = 0.0068 ( 9 ) , ( f B 0 − f B + ) / f B = 0.0047 ( 6 ) , and ( f B ⁎ 0 − f B ⁎ + ) / f B ⁎ = 0.0045 ( 5 ) , which yield: f D + − f D 0 = 0.97 ± 0.13 MeV , f D ⁎ + − f D ⁎ 0 = 1.73 ± 0.27 MeV , f B 0 − f B + = 0.90 ± 0.13 MeV , f B ⁎ 0 − f B ⁎ + = 0.81 ± 0.11 MeV . In the case of the D -meson our finding is consistent with recent lattice QCD results, whereas it is much lower in the case of the B -meson showing a tension of ≈3 standard deviations. [ABSTRACT FROM AUTHOR]

Abstract: The photon transition form factors of π, η and are discussed in view of recent measurements. It is shown that the exact axial anomaly sum rule allows a precise comparison of all three form factors at high- independent of the different structures and distribution amplitudes of the participating pseudoscalar mesons. We conclude: (i) The πγ form factor reported by Belle is in excellent agreement with the nonstrange component of the η and form factors obtained from the BaBar measurements. (ii) Within errors, the πγ form factor from Belle is compatible with the asymptotic pQCD behavior, similar to the η and form factors from BaBar. Still, the best fits to the data sets of πγ, ηγ, and form factors favor a universal small logarithmic rise . [Copyright &y& Elsevier]

Abstract: We present a sum-rule extraction of the decay constants of the charmed mesons D and 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 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 mass leads to a distinct hierarchy of the perturbative expansion. Furthermore, the decay constants extracted from the pole-mass correlator turn out to be considerably smaller than those obtained by means of the -mass correlator. Second, making use of the OPE in terms of the mass, we determine the decay constants of both D and mesons with an emphasis on the uncertainties in these quantities related both to the input QCD parameters and to the limited accuracy of the method of sum rules. [Copyright &y& Elsevier]

The generalization of the color gauge group SU(3) to SU (N c) , with N c taking arbitrarily large values, as had been proposed and developed by 't Hooft, has allowed for a decisive progress in the understanding of many qualitative, as well as quantitative, aspects of QCD in its nonperturbative regime. In particular, the notion of valence quarks receives there a precise meaning. The present work reviews the various aspects of the extension of this approach 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 with their singularities is highlighted. The main mechanisms of formation of tetraquark states, provided by the diquark model and the molecular scheme, are considered together with their specific implications. The properties of tetraquark states at large N c are analyzed through the Feynman diagrams that describe two-meson scattering amplitudes. It turns out that, in that limit, the possible formation of tetraquark states is mainly due to the mutual interactions of their internal mesonic clusters. These essentially arise from the quark-rearrangement, or quark-interchange, mechanism. In coupled-channel meson–meson scattering amplitudes, one may expect the occurrence of two independent tetraquark states, each having privileged couplings with the two mesons of their dominant channel. The question of the energy balance of various schemes in the static limit is also analyzed. The clarification of the mechanisms that are at work in the formation of tetraquarks is the main outcome from the large- N c approach to this problem. [ABSTRACT FROM AUTHOR]

The charmonium-like exotic states Y (4230) and the less known Y (4320) , produced in e + e − collisions, are sources of positive parity exotic hadrons in association with photons or pseudoscalar mesons. We analyze the radiative and pion decay channels in the compact tetraquark scheme, with a method that proves to work equally well in the most studied D ∗ → γ / π + D decays. The decay of the vector Y into a pion and a Z c state requires a flip of charge conjugation and isospin that is described appropriately in the formalism used. Rates are found to depend on the fifth power of pion momentum, which would make the final states π Z c (4020) strongly suppressed with respect to π Z c (3900) . The agreement with BES III data would be improved considering the π Z c (4020) events to be fed by the tail of the Y (4320) resonance under the Y (4230) . These results should renovate the interest in further clarifying the emerging experimental picture in this mass region. [ABSTRACT FROM AUTHOR]