On some implications of the BaBar data on the γ*η′ transition form factor

At leading-twist accuracy the form factors for the transitions from a virtual photon to the eta or eta' can be expanded into a power series of the variable omega, being related to the difference of the two photon virtualities. The series possess the remarkable feature that only the Gegenbauer coefficients of the meson distribution amplitudes of order l <= m contribute to the term similar to omega(m). Thus, for omega -> 0 only the asymptotic meson distribution amplitude contributes, allowing for a test of the mixing of the eta and eta' decay constants. Employing the Gegenbauer coefficients determined in an analysis of the form factors in the real photon limit, we present predictions for the gamma*eta and gamma*eta' form factors and compare them to the BaBar data. (C) 2019 The Authors. Published by Elsevier B.V.