University of Costa Rica

About: University of Costa Rica is a education organization based out in San José, Costa Rica. It is known for research contribution in the topics: Population & Venom. The organization has 9817 authors who have published 16781 publications receiving 238208 citations. The organization is also known as: UCR & Universidad de Costa Rica.

Evolved QCD predictions for the meson-photon transition form factors

Abstract: The QCD evolution of the pion distribution amplitude (DA) ${\ensuremath{\phi}}_{\ensuremath{\pi}}(x,{Q}^{2})$ 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 ${\ensuremath{\pi}}^{0}$, $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ using the hard-scattering formalism. We point out that a widely-used approximation of replacing $\ensuremath{\phi}(x,(1\ensuremath{-}x)Q)$ with $\ensuremath{\phi}(x,Q)$ 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 ${Q}^{2}$ dependence of the meson-photon transition form factors in the region of ${Q}^{2}g30\text{ }\text{ }{\mathrm{GeV}}^{2}$. 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 ${\ensuremath{\gamma}}^{*}\ensuremath{\gamma}\ensuremath{\rightarrow}{z}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}$ at leading twist. Our analysis thus indicates the importance of additional measurements of the pion-photon transition form factor at large ${Q}^{2}$.