Form factor of the process \gamma^*\gamma^* -->\pi^o for small Virtuality of One of the Photons and QCD Sum Rules

Abstract

We extend the QCD sum rule analysis of the \gamma^*\gamma^* -->\pi^o form factor into the region where one of the photons has small virtuality: q^2 << Q^2 > 1 GeV^2. In this kinematics, one should perform an additional factorization of short- and long-distance contributions. The extra long-distance sensitivity of the three-point amplitude is described by two-point correlators (bilocals), and the low-momentum dependence of the correlators involving composite operators of two lowest twists is extracted from auxiliary QCD sum rules. Our estimates for the form factor at q^2=0, Q^2 > 1 1 GeV^2 are in good agreement with existing experimental data.