Estimates for Pion-Photon Transition Distributions
  In the scaling regime, amplitudes for backward virtual Compton scattering and hadron annihilation into two photons depend on structure functions that describe the hadron-photon transition at the partonic level. We construct simple analytical models for the vector and axial-vector pion-photon transition distribution amplitudes based on double distributions. Via a sum rule, these models are tuned to reproduce the pion-photon transition form factors. To obtain reasonable estimates of the distributions at a low scale, we modify the models to saturate the positivity bounds using empirically parametrized parton distributions. We also determine a model-independent contribution to the axial-vector transition distribution using chiral perturbation theory. 
B. C. Tiburzi