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arXiv · 1105.2753

Pion-photon transition---the new QCD frontier

A. P. Bakulev·S. V. Mikhailov·A. V. Pimikov·N. G. Stefanis
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Abstract

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 NLO QCD radiative corrections and the twist-four contributions. The NNLO 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 $a_2$ and $a_4$, we show that the $1σ$ error ellipse of all data up to 9 GeV$^2$ 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 $(a_2,a_4)$ plane when including $a_6$. We argue that it is not possible to accommodate the high-$Q^2$ 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.

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A. P. Bakulev, S. V. Mikhailov, A. V. Pimikov, N. G. Stefanis. 2011-07-18. Pion-photon transition---the new QCD frontier. https://doi.org/10.1103/physrevd.84.034014

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