Evolved QCD predictions for the meson-photon transition form factors

The QCD evolution of the pion distribution amplitude (DA) phi(pi)(x, Q(2)) is computed for several commonly used models. Our analysis includes the nonperturbative form predicted by light-front holographic QCD, thus combining the nonperturbative bound state dynamics of the pion with the perturbative ERBL evolution of the pion DA. We calculate the meson-photon transition form factors for the pi(0), eta and eta' using the hard-scattering formalism. We point out that a widely-used approximation of replacing phi(x, (1 - x)Q) with phi(x, Q) in the calculations will unjustifiably reduce the predictions for the meson-photon transition form factors. It is found that the four models of the pion DA discussed give very different predictions for the Q2 dependence of the meson-photon transition form factors in the region of Q(2) > 30 GeV2. More accurate measurements of these transition form factors at the large Q(2) region will be able to distinguish different models of the pion DA. The rapid growth of the large Q(2) data for the pion-photon transition form factor reported by the BABAR Collaboration is difficult to explain within the current framework of QCD. If the BABAR data for the pion-photon transition form factor is confirmed, it could indicate physics beyond-the-standard model, such as a weakly coupled elementary C = + axial vector or pseudoscalar z(0) in the few GeV domain, an elementary field which would provide the coupling gamma*gamma -> z(0) -> pi(0) at leading twist. Our analysis thus indicates the importance of additional measurements of the pion-photon transition form factor at large Q(2).