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Updated predictions for the decay constants of the D, Ds, B and Bs mesons obtained from Borel QCD sum rules for heavy-light currents are presented and compared with the recent lattice averages performed by the Flavor Lattice Averaging Group. An excellent agreement is obtained in the charm sector, while some tension is observed in the bottom sector. Moreover, available lattice and QCD sum-rule calculations of the decay constants of the vector D*, Ds*, B* and Bs* mesons are compared. Again some tension in the bottom sector is observed., Proceedings of the 8th International Workshop on the CKM Unitarity Triangle (CKM 2014), Vienna, Austria, September 8-12, 2014

We present a sum-rule calculation of the decay constants of the charmed vector mesons $D^*$ and $D^*_s$ 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 $\overline{\rm MS}$ mass leads to a distinct hierarchy. Second, making use of the operator product expansion in terms of the $\overline{\rm MS}$ mass, we determine the decay constants of the $D^*$ and $D^*_s$ 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 $f_{D^*}=(252.2 \pm 22.3_{\rm OPE}\pm 4_{\rm syst})\; {\rm MeV}$ and $f_{D_s^*}=(305.5 \pm 26.8_{\rm OPE}\pm 5_{\rm syst})\; {\rm MeV}$. For the ratios of the vector-to-pseudoscalar decay constants we report $f_{D^*}/f_D= 1.221\pm 0.080_{\rm OPE}\pm 0.008_{\rm syst}$ and $f_{D^*_s}/f_{D_s}= 1.241\pm 0.057_{\rm OPE}\pm 0.007_{\rm syst}$., Comment: 9 pages, published version

We prove that Borel QCD sum rules for heavy-light currents yield very strong correlations between the $b$-quark mass $m_b$ and the $B$-meson decay constant $f_B$, namely, $\delta f_B/f_B\approx -8\,\delta m_b/m_b$. This fact opens the possibility of an accurate sum-rule extraction of $m_b$ by using $f_B$ as input. Combining precise lattice QCD determinations of $f_B$ with our sum-rule analysis based on the three-loop $O(\alpha_s^2)$ heavy-light correlation function leads to $\bar{m}_b(\bar{m}_b)=(4.247\pm0.034)\;\mbox{GeV}$., Comment: 11 pages

By means of QCD sum rules in local-duality limit, we analyze the behaviour of the form factors for the transitions of a real and a virtual photon to some pseudoscalar meson as functions of the involved momentum transfer. Except for the findings of BaBar for the neutral-pion form factor, the experimental data for all these transition form factors are compatible with saturation for large momentum transfer predicted by pQCD factorization. For light pseudoscalar mesons, saturation is observed already at relatively small momentum transfer, whereas for the eta-c meson it sets in only at larger momentum transfer. A recent measurement of the neutral-pion transition form factor by Belle seems to resolve this disturbing puzzle as its outcome is compatible with both saturation for relatively small momentum transfer and the behaviour of the eta and eta' transition form factors at large momentum transfer., 7 pages, 3 figures, talk presented at "Quark Confinement and the Hadron Spectrum X" (8 - 12 October 2012, TUM Campus Garching, Munich, Germany), to appear in the proceedings

The photon transition form factors of $\pi$, $\eta$ and $\eta'$ 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-$Q^2$ independent of the different structures and distribution amplitudes of the participating pseudoscalar mesons. We conclude: (i) The $\pi\gamma$ form factor reported by Belle is in excellent agreement with the non-strange I=0 component of the $\eta$ and $\eta'$ form factors obtained from the BaBar measurements. (ii) Within errors, the $\pi \gamma$ form factor from Belle is compatible with the asymptotic pQCD behavior, similar to the $\eta$ and $\eta'$ form factors from BaBar. Still, the best fits to the data sets of $\pi\gamma$, $\eta\gamma$, and $\eta'\gamma$ form factors favor a universal small logarithmic rise $Q^2 F_{P\gamma}(Q^2)\sim \log(Q^2)$., Comment: 5 pages

The surprising results by the BarBar collaboration on the $\pi\gamma$ transition form factor require new thoughts about the high-$Q^2$ dependence of the form factors with virtual photons. We make use of the anomaly sum rule [J. Horejsi and O. Teryaev, Z. Phys. C65, 691 (1995).] which relates the hadron spectral density to the axial anomaly [S. Adler, Phys. Rev. 177, 2426 (1969); J. S. Bell and R. Jackiw, Nuovo Cimento A 60, 47 (1969).]. We study the quark-hadron duality relation for this sum rule and find out that the increase of the rescaled form factor $Q^2F_{\pi\gamma}(Q^2)\sim\log(Q^2)$ suggested by the BaBar data requires the presence of a $1/s$-correction term in the relation between the one-loop spectral density and the hadron-continuum spectral density., Comment: 4 pages

The local-duality formulation of QCD sum rules allows for the prediction of hadronic form factors without knowledge of the subtle details of their structure. With the aid of this formalism, we take a fresh look at the behaviours of the charged-pion elastic form factor and of the form factors entering in the transitions of the ground-state neutral unflavoured pseudoscalar mesons pi^0, eta, eta' to one real and one virtual photon within a broad range of momentum transfers Q^2. The uncertainties induced by the approximations inherent to this local-duality approach are estimated by studying, in parallel to QCD, quantum-mechanical potential models, where the exact form factors, obtained by solving the Schr\"odinger equation, may be compared with the corresponding local-duality sum-rule results. For Q^2 larger than 5-6 GeV^2, we judge the predictions of the simplest local-duality model to be reliable and expect their accuracy to improve very fast with increasing Q^2. The large-Q^2 prediction for the pion elastic form factor should be approached already at moderate momentum transfer squared of approximately 4-8 GeV^2; large deviations from its local-duality behaviour for Q^2 = 20-50 GeV^2, predicted by some hadron-structure models, seem rather unlikely. The (eta, eta') to gamma + gamma* form factors deduced from the simplest local-duality approach exhibit excellent agreement with experiment. In startling contrast, BaBar measurements of the pi^0 to gamma + gamma* form factor imply local-duality violations which even rise with Q^2., Comment: 8 pages, talk presented at the "XXth International Workshop on High Energy Physics and Quantum Field Theory", 24 September - 1 October 2011, Sochi, Russia, to appear in the proceedings

We analyze $F_\pi(Q^2)$ and $F_{P\gamma}(Q^2)$, $P=\pi,\eta,\eta'$, within the local-duality (LD) version of QCD sum rules, which allows one to obtain predictions for hadron form factors in a broad range of momentum transfers. To probe the accuracy of this approximate method, we consider, in parallel to QCD, a potential model: in this case, the exact form factors may be calculated from the solutions of the Schr\"odinger equation and confronted with the results from the quantum-mechanical LD sum rule. On the basis of our quantum-mechanical analysis we conclude that the LD sum rule is expected to give reliable predictions for $F_\pi(Q^2)$ and $F_{\pi\gamma}(Q^2)$ in the region $Q^2 \ge 5-6$ GeV$^2$. Moreover, the accuracy of the method improves rather fast with growing $Q^2$ in this region. For the pion elastic form factor, the data at small $Q^2$ indicate that the LD limit may be reached already at relatively low values of momentum transfers, $Q^2\approx 4-8$ GeV$^2$; we therefore conclude that large deviations from LD in the region $Q^2=20-50$ GeV$^2$ reported in some recent theoretical analyses seem unlikely. The data on the ($\eta,\eta')\to\gamma\gamma^*$ form factors meet very well the expectations from the LD model. Surprisingly, the {\sc BaBar} results for the $\pi^0\to\gamma\gamma^*$ form factor imply a violation of LD growing with $Q^2$ even at $Q^2\approx 40$ GeV$^2$, at odds with the $\eta,\eta'$ case and the experience from quantum mechanics., Comment: 8 pages

We study the $P\to\gamma\,\gamma^*$ ($P=\pi^0,\eta,\eta'$) transition form factors by means of the local-duality (LD) version of QCD sum rules. For the case of $\eta$ and $\eta'$, the conventional LD model provides a good description of the existing data. However, for the $\pi$ form factor we find disagreement with recent BaBar results for high $Q^2$ even though the accuracy of the LD approximation is expected to increase with $Q^2$. It remains mysterious why the $\eta$ and $\eta'$ form factors to virtual photons, on the one hand, and the $\pi$ form factor, on the other hand, show a qualitatively different behaviour corresponding to a rising with $Q^2$ violation of local duality in the pion case. In a quantum mechanical example we show that, for a bound-state size of about 1 fm, the LD sum rule provides an accurate prediction for the form factor for $Q^2\ge$ a few GeV$^2$., Comment: 8 pages

Slight sophistications of the QCD sum-rule formalism may have great impact on the reliability of predicted hadron observables, as exemplified for the case of heavy-meson decay constants., 4 pages, 1 figure, talk presented at the "XIV International Conference on Hadron Spectroscopy", 13 - 17 June 2011, Munich, Germany, to appear in the proceedings

Dispersive sum rules constitute long-standing tools for extracting hadron features from QCD. We estimate the systematic uncertainties induced by assuming quark-hadron duality and improve the accuracy of the resulting predictions by elevating the effective thresholds involved in this approximation from constants to functions of momenta and a parameter entering upon Borel transformation. A rigorous scrutiny of the applicability of our proposed sum-rule modifications to QCD gives us great confidence that our sum-rule alterations will prove to be fruitful for hadron phenomenology., contributed to the "XIV International Conference on Hadron Spectroscopy", 13 - 17 June 2011, Munich, Germany, to appear in the proceedings

We present a sum-rule extraction of the decay constants of heavy-light mesons from the two-point correlator of pseudoscalar currents. To this end, we compare the perturbative expansions for the correlator and the decay constant performed in terms of either the pole mass or the running MS-bar mass of the heavy quark. The perturbative expansion expressed in terms of the pole mass exhibits no sign of convergence whereas reorganizing this very expansion in terms of the MS-bar mass yields a rather clear hierarchy of the perturbative contributions. Accordingly, the decay constants extracted from the pole-mass correlator turn out to be considerably smaller than those extracted from its MS-bar-mass counterpart. Then, making use of the OPE in terms of the MS-bar mass we derive the decay constants of heavy mesons with emphasis on acquiring control over the uncertainties in the decay constants, related both to the input QCD parameters and to the limited accuracy of the method of sum rules. Gaining this control has become possible due to the application of our novel procedure for extracting hadron observables based on dual thresholds which depend on the Borel parameter., talk presented at the "XIXth International Workshop on High Energy Physics and Quantum Field Theory - QFTHEP'10", 8 - 15 September 2010, Golitsyno, Moscow, Russia

We study the forward-backward and the CP-violating asymmetries (both time-independent and time-dependent) in rare semileptonic $B_d\to\rho^0 l^+l^-$, $B_s\to\phi l^+l^-$, and radiative leptonic $B_{d,s}\to \gamma l^+l^-$ decays and investigate the sensitivity of these asymmetries to the extensions of the Standard model., Comment: Revised version: plots for A_CP corrected, discussion of B_d\to \rho^0 l^+l^- added

We present our recent results for the pion elastic form factor [1] obtained within a local-duality three-point sum rule (a Borel sum rule in the limit of an infinite Borel parameter). Our analysis includes the O(1) and O(\alpha_s) contributions and is therefore applicable in a broad range of spacelike momentum transfers. Our results demonstrate in essentially model-independent way that the O(1) term, which provides the subleading 1/Q^4 power correction at asymptotically large momentum transfers, contributes more than half of the pion form factor in the region Q^2 \le 20 GeV^2. To probe the accuracy of local-duality sum rules for form factors, we apply precisely the same procedures to extract the form factor in a quantum-mechanical potential model. Comparison of the exact form factor known in this model with the result of the sum-rule calculation gives a probe of the systematic error of the method. In our example this error is found to be at the level of 10-20%. We expect similar systematic errors for form factors obtained from local-duality sum rules in QCD., Comment: Talk given at the 8-th Conference "Quark Confinement and the Hadron Spectrum", Mainz, Germany, 1-6 September 2008

We present the results of our recent systematic study of the light-cone expansion of heavy-to-light transition form factors in a model with scalar constituents [1]. We show that the higher-twist contributions (represented in this model by off-light-cone effects) all have the same behaviour in the 1/m_Q expansion. The suppression parameter of the higher-twist contributions compared to the lower-twist contributions is, in general, the inverse Borel parameter \beta. The only exception here is the case of the leading and the subleading twists: they are of the same order in 1/\beta because of an extra suppression of the leading-twist contribution to the form factor., Comment: 3 pages, Talk given at XIII International Conference "Selected Problems of Modern Theoretical Physics", Dubna, Russia, June 23-27, 2008

This talk presents the results of our study of systematic errors of the ground-state parameters obtained by Shifman-Vainshtein-Zakharov (SVZ) sum rules. We use the harmonic-oscillator potential model as an example: in this case we know the exact solution for the polarization operator, which allows us to obtain both the OPE to any order and the parameters (masses and decay constants) of the bound states. We extract the parameters of the ground state by making use of the standard procedures of the method of QCD sum rules, and compare the obtained results with their known exact values. We show that if the continuum contribution to the polarization operator is not known and is modelled by some effective continuum threshold, the standard procedures adopted in sum rules do not allow one to gain control over the systematic errors of the extracted ground-state parameters., 8 pages, talk given at "Hadron 07", XII International Conference on Hadron Spectroscopy, Frascati, October 8-13, 2007

Long-distance QCD effects in $B_{d,s}\to\ell^+\ell^-\gamma$ decays are analyzed. We show that the contribution of the light vector-meson resonances related to the virtual photon emission from valence quarks of the $B$-meson, which was not considered in previous analyses, strongly influences the dilepton differential distrubution. Taking into account photon emission from the $b$-quark loop, weak annihilation, and Bremsstrahlung from leptons in the final state, we give predictions for dilepton spectrum in $B_{d, s}\to \ell^+\ell^-\gamma$ decays within the Standard model., Comment: 8 pages, typoes corrected, replaced with the version to appear in Physical Review D

We analyse the form factors for the B --> gamma l^+l^- weak transition. We show that making use of the gauge invariance of the B --> gamma l^+l^- amplitude, the structure of the form factors in the resonance region, and their relations at large values of the photon energy results in efficient constraints on the behavior of the form factors. Based on these constraints, we propose a simple parametrization of the form factors and apply it to the lepton forward-backward (FB) asymmetry in the B --> gamma l^+l^- decay. We find that the behavior of the FB asymmetry as a function of the photon energy, as well as the location of its zero, depend only weakly on the B --> gamma l^+l^- form factors, and thus constitutes a powerful tool for testing the standard model., Comment: 10 pages, ReVTeX4, 2 figures. Version to appear in Phys. Rev. D, with some minor additions

We analyse the corrections to factorization for the $B-\bar B$ mixing amplitude assuming that non-factorizable soft gluon exchanges can be described by the local gluon condensate. Within this approximation the $$-correction to the mixing amplitude can be expressed through the $B$-meson transition form factors at zero momentum transfer. The correction is negative independent of the particular values of the form factors. As a next step, we study these form factors making use of the relativistic dispersion approach based on the constituent quark picture. We obtain spectral representations for the form factors in terms of the $B$-meson soft wave function and the propagator of the color-octet $b\bar q$ system. The $B$-meson wave function has been determined previously from weak exclusive B decays, so the main uncertainty in our estimates comes from the unknown details of the propagator in the nonperturbative region. We obtain $\Delta B_{B_d}(m_b)=-0.06\pm 0.035 $ and $\Delta B_{B_s}(m_b)=-0.05\pm 0.03 $., Comment: revtex, 8 pages

Nonperturbative effects in exclusive reactions $B\to K^{(*)}ll$ are discussed. Form factors which describe the main long-distance contribution to the meson transition amplitude are calculated within the dispersion formulation of the quark model: namely, the form factors in the decay region are expressed as relativistic double spectral representations through the wave functions of the initial and final mesons. The dilepton forward-backward asymmetry and the lepton polarization turn out to be largely independent of the particular choice of the quark model parameters and can be predicted with high accuracy. At the same time, these asymmetries are sensitive to the short-distance structure of the theory and might be used as a test of the Standard Model and probe of new physics., talk given at XXXIInd Rencontres de Moriond 'Electroweak Interactions', Les Arcs, France, March 1997, 4 pages, 2 figs