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Abstract We revisit the calculation of nonfactorizable corrections induced by charm-quark loops in exclusive FCNC B -decays. For the sake of clarity, we make use of a field theory with scalar particles: this allows us to focus on the conceptual issues and to avoid technical complications related to particle spins in QCD. We perform a straightforward calculation of the appropriate correlation function and show that it requires the knowledge of the full generic three-particle distribution amplitude with non-aligned arguments, 〈 0 | s ¯ (y) G μ ν (x) b (0) | B (p) 〉. Moreover, the dependence of this quantity on the variable (x − y) 2 is essential for a proper account of the (Λ QCD m b / m c 2) n terms in the amplitudes of FCNC B -decays. [ABSTRACT FROM AUTHOR]

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]

The R K measurement by LHCb suggests non-standard lepton non-universality (LNU) to occur in b → s ℓ + ℓ − transitions, with effects in muons rather than electrons. A number of other measurements of b → s ℓ + ℓ − transitions by LHCb and B -factories display disagreement with the SM predictions and, remarkably, these discrepancies are consistent in magnitude and sign with the R K effect. Non-standard LNU suggests non-standard lepton flavor violation (LFV) as well, for example in B → K ℓ ℓ ′ and B s → ℓ ℓ ′ . There are good reasons to expect that the new effects may be larger for generations closer to the third one. In this case, the B s → μ e decay may be the most difficult to reach experimentally. We propose and study in detail the radiative counterpart of this decay, namely B s → μ e γ , whereby the chiral-suppression factor is replaced by a factor of order α / π . A measurement of this mode would be sensitive to the same physics as the purely leptonic LFV decay and, depending on experimental efficiencies, it may be more accessible. A realistic expectation is a factor of two improvement in statistics for either of the B d , s modes. [ABSTRACT FROM AUTHOR]

We present a sum-rule calculation of the decay constants of the charmed vector mesons D⁎ and Ds⁎ from the two-point correlator of vector currents. First, we show that the perturbative expansion in terms of the pole mass exhibits no sign of convergence whereas the reorganization of this expansion in terms of the MS¯ mass leads to a distinct hierarchy. Second, making use of the operator product expansion in terms of the MS¯ mass, we determine the decay constants of the D⁎ and Ds⁎ mesons with an emphasis on the uncertainties in these theoretically predicted quantities related both to the input QCD parameters and to the limited accuracy of the method of sum rules. Our results are fD⁎=(252.2±22.3OPE±4syst) MeV and fDs⁎=(305.5±26.8OPE±5syst) MeV. For the ratios of the vector-to-pseudoscalar decay constants we report fD⁎/fD=1.221±0.080OPE±0.008syst and fDs⁎/fDs=1.241±0.057OPE±0.007syst. [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]

Abstract: We compare the extraction of the ground-state decay constant from the two-point correlator in QCD and in potential models and show that the results obtained at each step of the extraction procedure follow a very similar pattern. We prove that allowing for a Borel-parameter-dependent effective continuum threshold yields two essential improvements compared to employing a Borel-parameter-independent quantity: (i) It reduces considerably the (unphysical) dependence of the extracted bound-state mass and the decay constant on the Borel parameter. (ii) In a potential model, where the actual value of the decay constant is known from the Schrödinger equation, a Borel-parameter-dependent threshold leads to an improvement of the accuracy of the extraction procedure. Our findings suggest that in QCD a Borel-parameter-dependent threshold leads to a more reliable and accurate determination of bound-state characteristics by the method of sum rules. [Copyright &y& Elsevier]

Abstract: We analyse the extraction of the bound-state form factor from vacuum-to-hadron correlator, which is the basic object for the calculation of hadron form factors in the method of light-cone sum rules in QCD. We study this correlator in quantum mechanics, calculate it exactly, and derive the corresponding OPE. We then apply the standard procedures of QCD sum rules to isolate the ground-state form factor from this correlator. We demonstrate that fixing the effective continuum threshold, one of the key ingredients of the sum-rule calculation of bound-state parameters, poses a serious problem for sum rules based on vacuum-to-hadron correlators. [Copyright &y& Elsevier]

Abstract: We discuss the extraction of form factors from three-point sum rules making use of a harmonic-oscillator model, where we derive the exact expression for the relevant correlator. We determine the form factor of the ground state by the standard procedures adopted in the method of sum rules, and compare the obtained results with the known exact values. We show that the uncontrollable uncertainty in the extracted value of the form factor is typically much larger than that for the decay constant. In the example considered, we find the uncontrolled systematic error in the extracted form factor to exceed the 10% level. [Copyright &y& Elsevier]

Abstract: We study the possibility to control 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 QCD 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 errors of the extracted ground-state parameters. [Copyright &y& Elsevier]

It is shown that the decay amplitude for the Jaffe-Wilczek type pentaquarks is not suppressed by chiral symmetry. On the other hand, pentaquarks of positive or negative parity built up of an antiquark and two chirally different diquarks in an S-state are stable in the limit of a strictly unbroken chiral symmetry. These states can decay only via the spontaneous breaking of chiral symmetry. However, this breaking is strong because of the sizeable magnitude of the quark condensate. Thus, chiral symmetry cannot be the cause of a tiny decay amplitude, even for pentaquarks which are stable in the strict chiral symmetry limit. [Copyright &y& Elsevier]

Abstract: We demonstrate that a five-quark state of positive parity with an internal P-wave structure—usually pictured as a composite of an antiquark and two diquarks in a P-wave state—can couple to nucleons and Goldstone particles in a chirally invariant way. The corresponding decay width is generally not suppressed. A pentaquark of positive or negative parity with an internal S-wave structure, which may be viewed as a composite of an antiquark and two chirally different diquarks in an S-state, does not couple to nucleons and light mesons in the limit of an unbroken chiral symmetry. It is stable in this limit. However, such states can decay via the effect of the spontaneous breaking of chiral symmetry. This breaking is strong because of the sizable magnitude of the quark condensate. Thus, chiral symmetry cannot be the cause of a tiny decay amplitude, even for pentaquarks stable in a strict chiral symmetry limit. [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]

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]