The asymptotics of the transition form factor γγ* -> piᵒ and QCD sum rules

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In this paper we present the result of a direct QCD sum rule calculation of the transition form factor γγ* -> piᵒ in the region of moderately large invariant momentum Q² > 1GeV² of the virtual photon. In contrast to pQCD, we make no assumptions about the shape of the pion distribution amplitude ϕπ(x). Our results agree with the Brodsky-Lepage proposal that the Q²-dependence of this form factor is given by an interpolation between its Q²=0 value fixed by the axial anomaly and 1/Q² pQCD behaviour for large Q², with normalization corresponding to the asymptotic form ϕπᵃs (x)=6 fπx (1-x) of the pion distribution amplitude. Our prediction for the from factor Fᵧ*ᵧ*π∘(q₁² = 0,q₂² = -Q²) is in good agreement with new CLEO data.