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A state-of-the-art analysis of the pion-photon transition form factor is presented based on an improved theoretical calculation that includes the effect of a finite virtuality of the quasireal photon in the method of light-cone sum rules. We carry out a detailed statistical analysis of the existing experimental data using this method and by employing pion distribution amplitudes with up to three Gegenbauer coefficients a2, a4, a6. Allowing for an error range in the coefficient a6 ≈ 0, the theoretical predictions for γ*γ → π0 obtained with nonlocal QCD sum rules are found to be in good agreement with all data that support a scaling behavior of the transition form factor at higher Q², like those of the Belle Collaboration. The data on γ*γ → η/η' from CLEO and BABAR are also reproduced, while there is a strong conflict with the auxetic trend of the BABAR data above 10 GeV². The broader implications of these findings are discussed. [ABSTRACT FROM AUTHOR]

We discuss cross-link relations between the π and ρ-meson channels emerging from two different descriptions of the QCD vacuum: instanton physics and QCD sum rules with nonlocal condensates (NLCs). We derive in both schemes an intriguing linear relation between the π and the ρ∥-meson distribution amplitudes in terms of their conformal coefficients and work out the specific impact of the scalar NLC in these two channels. Using a simple model with Gaussian decay of the scalar NLC, we are able to relate it to the moments of the pion nonsinglet parton distribution function measurable in experiment--a highly nontrivial result. The implications for the pion and the ρ∥-meson distribution amplitudes entailed by the obtained cross-link relations are outlined in terms of two generic scenarios. [ABSTRACT FROM AUTHOR]

We study the pion-photon transition form factor (TFF) Fγ*γπ0(Q²) using a state-of-the art implementation of light cone sum rules (LCSRs) within fixed-order QCD perturbation theory. The spectral density in the dispersion relation includes all currently known radiative corrections up to the next-to-next-to-leading-order (NNLO) and all twist contributions up to order six. Predictions for the TFF are obtained for various pion distribution amplitudes (DAs) of twist two, including two-loop evolution which accounts for heavy-quark mass thresholds. The influence of the main theoretical uncertainties is quantified in order to enable a more realistic comparison with the data. The characteristics of various pion DAs are analyzed in terms of the conformal coefficients a2 and a4 in comparison with the 1σ and 2σ error regions of the data and the most recent lattice constraints on a2 with NLO and NNLO accuracy. Our results provide more stringent bounds on the variation of the pion DA and illuminate the corresponding asymptotic behavior of the calculated TFF. [ABSTRACT FROM AUTHOR]

We show that using renormalization-group summation to generate the QCD radiative corrections to the π-γ transition form factor, calculated with light-cone sum rules (LCSR), renders the strong coupling free of Landau singularities while preserving the QCD form-factor asymptotics. This enables a reliable applicability of the LCSR method to momenta well below 1 GeV². This way, one can use the new preliminary BESIII data with unprecedented accuracy below 1.5 GeV² to fine tune the prefactor of the twist-six contribution. Using a combined fit to all available data below 3.1 GeV², we are able to determine all nonperturbative scale parameters and a few Gegenbauer coefficients entering the calculation of the form factor. Employing these ingredients, we determine a pion distribution amplitude with conformal coefficients (b2,b4) that agree at the 1σ level with the data for Q²≤3.1 GeV² and fulfill at the same time the lattice constraints on b2 at N³LO together with the constraints from QCD sum rules with nonlocal condensates. The form-factor prediction calculated herewith reproduces the data below 1 GeV² significantly better than analogous predictions based on a fixed-order power-series expansion in the strong coupling constant. [ABSTRACT FROM AUTHOR]

We consider the light cone sum-rule description of the pion-photon transition form factor, based on dispersion relations, in combination with the renormalization group of QCD, in terms of the formal solution of the Efremov-Radyushkin-Brodsky-Lepage evolution equation, and show that the emerging scheme amounts to a certain version of fractional analytic perturbation theory (FAPT). In order to ensure the correct asymptotic behavior of the considered physical quantity, this modified FAPT version has to be supplemented by process-specific boundary conditions--in contrast to the standard one. However, it provides the advantage of significantly improving the inclusion of radiative corrections in the low-momentum regime of QCD perturbation theory using renormalization-group summation. [ABSTRACT FROM AUTHOR]

We perform a comparative theoretical study of the data at spacelike momentum transfer for the y*y →π0 transition form factor, just reported by the Belle Collaboration, vs. those published before by BABAR, also including the older CLEO and CELLO data. Various implications for the structure of the 77° distribution amplitude vis-à-vis those data are discussed and the existing theoretical predictions are classified into three distinct categories. We argue that the actual bifurcation of the data with antithetic trends is artificial and reason that the Belle data are the better option. [ABSTRACT FROM AUTHOR]

We perform a detailed analysis of all existing data (CELLO, CLEO, BABAR) on the pion-photon transition form factor by means of light-cone sum rules in which we include the next-to-leading order QCD radiative corrections and the twist-four contributions. The next-to-next-to-leading order radiative correction together with the twist-six contribution are also taken into account in terms of theoretical uncertainties. Keeping only the first two Gegenbauer coefficients a2 and a4, we show that the 1α error ellipse of all data up to 9 GeV² greatly overlaps with the set of pion distribution amplitudes obtained from nonlocal QCD sum rules--within the range of uncertainties due to twist-four. This remains valid also for the projection of the 1σ error ellipsoid on the (a2, a4) plane when including a6. We argue that it is not possible to accommodate the high-Q² tail of the BABAR data with the same accuracy, despite opposite claims by other authors, and conclude that the BABAR data still pose a challenge to QCD. [ABSTRACT FROM AUTHOR]