Two-gluon components of theηandη′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 ${\eta }^{\prime }$ mesons and establish a consistent set of conventions for the definition of the gluon distribution amplitude, the anomalous dimensions, as well as the projector of a two-gluon state onto an $\eta$ or ${\eta }^{\prime }$ state. We calculate the $\eta ,$ ${\eta }^{\prime }$-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 $\eta ,$ ${\eta }^{\prime }$-photon transition form factor data. In order to elucidate the role of the two-gluon Fock component further, we analyze electroproduction of $\eta ,{\eta }^{\prime }$ mesons and the $g^{*}{g}^{*}\eta \left({\eta }^{\prime }\right)$ vertex.