A puzzle of the pion-photon transition form factor - resolved?

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.

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