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

Abstract

At leading-twist accuracy the form factors for the transitions from a virtual photon to the η or η′ can be expanded into a power series of the variable ω, 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 ∼ωm. Thus, for ω→0 only the asymptotic meson distribution amplitude contributes, allowing for a test of the mixing of the η and η′ decay constants. Employing the Gegenbauer coefficients determined in an analysis of the form factors in the real photon limit, we present predictions for the γ⁎η and γ⁎η′ form factors and compare them to the BaBar data. Keywords: Hard exclusive processes, Perturbative QCD, Meson transition form factors, Eta mesons