Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: QCD Sum Rule Calculation of $\gamma\gamma^*\to\pi^0$ Transition Form
  Factor   We develop a QCD sum rule analysis of the form factor
$F_{\gamma^*\gamma^*\pi^0}(q^2,Q^2)$ in the region where virtuality of one of
the spacelike photons is small $q^2 \ll 1 GeV^2$ 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
$\varphi_{\pi}^{as} (x)=6 f_{\pi} x (1-x)$.

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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: bayes-an
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The categories of this paper are: funct-an
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The categories of this paper are: econ
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The categories of this paper are: q-fin
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The categories of this paper are: adap-org
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The categories of this paper are: gr-qc
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The categories of this paper are: eess
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The categories of this paper are: q-bio
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The categories of this paper are: hep-ex
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The categories of this paper are: math-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-lat