γγ→ηc(1S,2S) transition form factors for spacelike photons

We derive the light-front wave function (LFWF) representation of the γ*γ*→ηc(1S),ηc(2S) transition form factor F(Q12,Q22) for two virtual photons in the initial state. For the LFWF, we use different models obtained from the solution of the Schrödinger equation for a variety of cc¯ potentials. We compare our results to the BABAR experimental data for the ηc(1S) transition form factor, for one real and one virtual photon. We observe that the onset of the asymptotic behavior is strongly delayed and discuss applicability of the collinear and/or massless limit. We present some examples of two-dimensional distributions for F(Q12,Q22). A factorization breaking measure is proposed and factorization breaking effects are quantified and shown to be almost model independent. Factorization is shown to be strongly broken, and a scaling of the form factor as a function of Q¯2=(Q12+Q22)/2 is obtained.