A perturbative approach to the ηcγ transition form factor

The ηcγ transition form factor is calculated within a perturbative approach. For the ηc-meson, a wave function of the Bauer-Stech-Wirbel type is used where the two free parameters, namely the decay constant fηc and the transverse size of the wave function, are related to the Fock state probability and the width for the two-photon decay Γ[ηc → γγ]. The Q2 dependence of the ηcγ transition form factor is well determined.