Twist-Six Corrections to $\eta \gamma$ and $\eta ^{'} \gamma$ Transition Form Factors in QCD

Abstract

Twist-six corrections to $\eta \gamma$ and $\eta ^{'}\gamma$ transition form factors have been calculated using collinear factorization approach. A comparison of the result with the data provides constraints on the lowest Gegenbauer coefficients of distribution amplitudes for the valence octet and singlet quark-antiquark and the gluon-gluon Fock components. For one of the photons approaching mass-shell, one encounters divergences which require judicious regularization.