QCD sum rule calculation of gamma gamma* to pi0 transition form-factor

while another is large: Q~2 \gapprox 1 GeV~2. We construct the operator product expansion suitable for this kinematic situation and obtain a QCD sum rule for

F_{\gamma~*\gamma~*\pi~0}(0,Q~2)

. Our results confirm expectation that the momentum transfer dependence of

F_{\gamma~*\gamma~*\pi~0}(0,Q~2)

is close to interpolation between its

Q~2=0

value fixed by the axial anomaly and

Q~{-2}

pQCD behaviour for large

Q~2

. Our approach, in contrast to pQCD, does not require additional assumptions about the shape of the pion distribution amplitude

\varphi_{\pi}(x)

. The absolute value of the

1/Q~2

term obtained in this paper favours

\varphi_{\pi}(x)

close to the asymptotic form

\varphi_{\pi}~{as} (x)=6 f_{\pi} x (1-x)

.

photon photon: exclusive reaction
photon photon: fusion
pi0: photoproduction
photoproduction: pi0
photon: off-shell
form factor: transition
transition: form factor
dependence: momentum transfer
quantum chromodynamics: sum rule
operator product expansion

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