ηc elastic and transition form factors: Contact interaction and algebraic model

For the flavor-singlet heavy-quark system of charmonia in the pseudoscalar [eta(c)(1S)] channel, we calculate the elastic (EFF) and transition form factors (TFFs) [eta(c)(1S)->gamma gamma*] for a wide range of photon momentum transfer squared (Q(2)). The framework for this analysis is provided by a symmetry-preserving Schwinger-Dyson equation and Bethe-Salpeter equation treatment of a vector x vector contact interaction. We also employ an algebraic model, developed earlier to describe the light-quark systems. It correctly correlates infrared and ultraviolet dynamics of quantum chromodynamics (QCD). The contact interaction results agree with the lattice data for low Q(2). For Q(2) >= Q(0)(2), the results start deviating from the lattice results by more than 20%. Q(0)(2) approximate to 2.5 GeV2 for the EFF, and approximate to 25 GeV2 for the TFF. We also present the results for the EFF, TFF, and eta(c)(1S) parton distribution amplitude for the algebraic model. Wherever the comparison is possible, these results are in excellent agreement with the lattice, perturbative QCD, results obtained through a Schwinger-Dyson equation-Bethe-Salpeter equation study, employing refined truncations, and the experimental findings of the BABAR experiment.