Measurements of the γγ＊ → π^0 and γγ＊ → ηc transition form factors at BABAR

We present measurements of the γγ＊ → π^0 transition form factor for the momentum transfer range Q^2=4-40 GeV^2 and the γγ＊ → ηc transition form factor for the range Q^2=2-50 GeV^2. The current status of measurements of the meson-photon transition form factors for the η and η＇ mesons is discussed. The results of the measurement of the ηc mass, total and two-photon widths are also presented.     

Calculation of the axial form factor of the decay π → evγ by means of QCD sum rules

The axial form factor of the decay π → evγ is calculated by means of QCD sum rules, using the experimental value for the mean- squared charge radius of the pion. The ratio of the axial to vector form factor is predicted to be γ = 0.42±0.2.     

Analysis of the decay B^0→Xc1π^0 with light-cone QCD sum rules

In this article, we calculate the contribution from the nonfactorizable soft hadronic matrix element to the decay B^0→Xc1π^0 with the light-cone quantum chromo-dynamic （QCD） sum rules. The numerical results show that its contribution is rather large and should not be neglected. The total amplitudes lead to a branching fraction which is in agreement with the experimental data marginally.     

Finite-Width Correction to Form Factors of γγ^＊→ π^0 Transition

Instead of the usual zero-width approximation for one resonance, we use the finite-width approximation for the two low resonances, i.e. the ρ- ω mesons, to investigate the light-cone local QCD sum rules for the form factor of the transition γγ^＊→ π^0 ,According to the method of the analytic continuation by duality, the weight function, the polynomial of a low order N, is added to the dispersion integral to annihilate the integrand in the region where both resonance saturation and the QCD asymptotic expression are least reliable. The resultant form factor in the cases for the zero- and finite-widths are almost the same, both agree well with the experimental measurements. A comparison with the result from the Laplacian transformed light-cone sum rules and a brief discussion are given.     

Analysis of the coupling constants ga0ηπ0 and ga0η＇ π0 with light-cone QCD sum rules

In this article, we take the point of view that the light scalar meson a0（980） is a conventional qqstate, and calculate the coupling constants ga0ηπ0 and ga0ηπ0 with the light-cone QCD sum rules. The central value of the coupling constant ga0ηπ0 is consistent with that extracted from the radiative decay φ（1020） → a0（980）γ→ηπ0γ. The central value and lower bound of the decay width Γa0→ηπ0 =127＋8448 MeV are compatible with the experimental data of the total decay width Γa0（980） = （50-100） MeV from the Particle Data Group with a very model dependent estimation （the decay width can be much larger）, while the upper bound is too large. We give a possible explanation for the discrepancy between the theoretical calculation and experimental data.     

Heavy-to-light transition form factors and their relations in light-cone QCD sum rules

The light-cone QCD sum rules approach improved by using the chiral current correlator is systematically reviewed and applied to the calculation of all the heavy-to-light form factors, including all the semileptonic and penguin ones. By choosing suitable chiral currents, the light-cone sum rules for all the form factors are greatly simplified and depend mainly on one leading-twist distribution amplitude of the light meson. As a result, relations between these form factors arise naturally. At the considered accuracy, these relations reproduce the results obtained in the literature. Moreover, since the explicit dependence on the leading-twist distribution amplitudes is preserved, these relations may be more useful to simulate the experimental data and extract information on the distribution amplitude.     

Dispersive methods and QCD sum rules for γ γ collisions

It has been shown, in the case of meson photoproduction, that the power-law falloff of these reactions can be described by lowest-order (real) sum rules, at moderate momentum transfer. The phases of these processes, in this regime, are usually thought to be non-perturbative. In a sum rule framework, however, they can possibly be described by radiative corrections to the hadronic spectral densities of the corresponding helicities, which become complex functions to order α_s, and the effects of interference can be strongly enhanced by the presence of the vacuum condensates in the dispersion relation. It is shown that the imaginary parts of these complex corrections have a factorized form and can be evaluated in a systematic fashion, while their real parts, at the same perturbative order, are down by at least two powers of momentum transfer. The analysis is done at two-loop level, combining dimensional regularization and light-cone methods. The calculations are performed for all the independent set of scalar diagrams generated by the OPE. The analytical bounds are identified and discussed.     

γ * ρ 0→π 0 transition form factor in extended AdS/QCD models

The γ ^* ρ ⁰→π ⁰ transition form factor is extracted from recent result for the γ ^* γ ^* π ⁰ form factor obtained in the extended hard-wall AdS/QCD model with a Chern–Simons term. In the large momentum region, the form factor exhibits a 1/Q ⁴ behavior, in accordance with the perturbative QCD analysis, and also with the Light-Cone Sum Rule (LCSR) result if the pion wave function exhibits the same endpoint behavior as the asymptotic one. The appearance of this power behavior from the AdS side and the LCSR approach seem to be rather similar: both of them come from the “soft” contributions. Comparing the expressions for the form factor in both sides, one can obtain the duality relation $z\propto \sqrt{u(1-u)}$ , which is compatible with one of the most important relations of the Light-Front holography advocated by Brodsky and de Teramond. In the moderate Q ² region, the comparison of the numerical results from both approaches also supports a asymptotic-like pion wave function, in accordance with previous studies for the γ ^* γ ^* π ⁰ form factor. The form factor at zero momentum transfer gives the γ ^* ρ ⁰ π ⁰ coupling constant, from which one can determine the partial width for the ρ ⁰(ω)→π ⁰ γ decay. We also calculate the form factor in the time-like region, and study the corresponding Dalitz decays ρ ⁰(ω)→ π ⁰ e ⁺ e ^?, π ⁰ μ ⁺ μ ^?. Although all these results are obtained in the chiral limit, numerical calculations with finite quark masses show that the corrections are extremely small. Some of these calculations are repeated in the Hirn–Sanz model and similar results are obtained.     

Analysis of the strong coupling form factors of Σ_bNB and Σ_cND in QCD sum rules

In this article, we study the strong interaction of the vertices Σ_bNB and Σ_cND using the three-point QCD sum rules under two different Dirac structures. Considering the contributions of the vacuum condensates up to dimension 5 in the operation product expansion, the form factors of these vertices are calculated. Then, we fit the form factors into analytical functions and extrapolate them into time-like regions, which gives the coupling constants. Our analysis indicates that the coupling constants for these two vertices are G_(Σ_bNB)= 0.43±0.01 GeV~(-1) and G_(Σ_cND)=3.76±0.05 GeV~(-1).     

Λb → Λc form factors from QCD light-cone sum rules

In this study, we calculate the transition form factors of \begin{document}$ \Lambda_b $\end{document} decaying into \begin{document}$ \Lambda_c $\end{document} within the framework of light-cone sum rules with the distribution amplitudes (DAs) of the \begin{document}$ \Lambda_b $\end{document}-baryon. In the hadronic representation of the correlation function, we isolate both the \begin{document}$ \Lambda_c $\end{document} and \begin{document}$ \Lambda_c^* $\end{document} states so that the \begin{document}$ \Lambda_b \rightarrow \Lambda_c $\end{document}form factors can be obtained without ambiguity. We investigate the P-type and A-type currents to interpolate light baryons for comparison because the interpolation current for the baryon state is not unique. We also employ three parametrization models for the DAs of \begin{document}$ \Lambda_b $\end{document} in the numerical calculation. We present the numerical predictions for the \begin{document}$ \Lambda_b \rightarrow \Lambda_c $\end{document} form factors and branching fractions, averaged forward-backward asymmetry, averaged final hadron polarization, and averaged lepton polarization of the \begin{document}$ \Lambda_b \to \Lambda_c \ell\mu $\end{document} decays, as well as the ratio of the branching ratios \begin{document}$ R_{\Lambda_c} $\end{document}. The predicted \begin{document}$ R_{\Lambda_c} $\end{document} is consistent with LHCb data.     

B → A transitions in the light-cone QCD sum rules with the chiral current

In this article, we calculate the form-factors of the transitions B → a₁(1260), b₁(1235) in the leading-order approximation using the light-cone QCD sum rules. In calculations, we choose the chiral current to interpolate the B-meson, which has the outstanding advantage that the twist-3 light-cone distribution amplitudes of the axial-vector mesons makes no contributions, and the resulting sum rules for the form-factors suffer from far fewer uncertainties. Then we study the semi-leptonic decays B → a₁(1260) lv_l, b₁(1235) lv_l (l=e,μ,τ), and make predictions for the differential decay widths and decay widths, which can be compared with the experimental data in the coming future.     

B → A transitions in the light-cone QCD sum rules with the chiral current

In this article, we calculate the form-factors of the transitions B → a₁(1260), b₁(1235) in the leading-order approximation using the light-cone QCD sum rules. In calculations, we choose the chiral current to interpolate the B-meson, which has the outstanding advantage that the twist-3 light-cone distribution amplitudes of the axial-vector mesons makes no contributions, and the resulting sum rules for the form-factors suffer from far fewer uncertainties. Then we study the semi-leptonic decays B → a₁(1260) lv_l, b₁(1235) lv_l (l=e,μ,τ), and make predictions for the differential decay widths and decay widths, which can be compared with the experimental data in the coming future.     

Analysis of the coupling constants g_(a_0ηπ~0) and g_(a_0η’ π~0) with light-cone QCD sum rules

In this article, we take the point of view that the light scalar meson a0(980) is a conventional qqstate, and calculate the coupling constants ga0ηπ0 and ga0ηπ0 with the light-cone QCD sum rules. The central value of the coupling constant ga0ηπ0 is consistent with that extracted from the radiative decay φ(1020) → a0(980)γ→ηπ0γ. The central value and lower bound of the decay width Γa0→ηπ0 =127+8448 MeV are compatible with the experimental data of the total decay width Γa0(980) = (50-100) MeV from the Particl...     

Form factors of γ * ρ → π and γ * γ → π 0 transitions and light-cone sum rules

The method of light-cone QCD sum rules is applied to the calculation of the form factors of and transitions. We consider the dispersion relation for the amplitude in the variable . At large virtualities and , this amplitude is calculated in terms of light-cone wave functions of the pion. As a next step, the light-cone sum rule for the form factor is derived. This sum rule, together with the quark-hadron duality, provides an estimate of the hadronic spectral density in the dispersion relation. Finally, the form factor is obtained taking the limit in this relation. Our predictions are valid at and have a correct asymptotic behaviour at large . Received: 16 January 1998 / Revised version: 14 May 1998 / Published online: 26 August 1998     

Determination of the constantg ωρπ from QCD sum rules

The ωρπ coupling constant is calculated using a modified form of sum rules for the vertex function 〈0|T(J _μ(x),J _ν(0))|π〉 accounting for the axial anomaly. The resultg _ωρπ=16 GeV^?1 is in good agreement with the estimates of the Vector Meson Dominance model. We show that the standard procedure gives forg _ωρπ a considerably smaller value compared to the experimental number.