The two-gluon components of the eta and eta' mesons to leading-twist   accuracy
  We critically reexamine the formalism for treating the leading-twist contributions from the two-gluon Fock components occurring in hard processes that involve eta and eta' mesons and establish a consistent set of conventions for the definition of the gluon distribution amplitudes, the anomalous dimensions as well as the projector of a two-gluon state onto an eta or eta' state. We calculate the eta, eta'--photon transition form factor to order alpha_s and show the cancellation of the collinear and UV singularities explicitly. An estimate of the lowest Gegenbauer coefficients of the gluon and quark distribution amplitudes is obtained from a fit to the eta, eta'--photon transition form factor data. In order to elucidate the role of the two-gluon Fock component further, we analyze electroproduction of eta, eta' mesons and the g*g* eta (eta') vertex. 
Peter Kroll and Kornelija Passek-Kumericki