Meson-Photon Transition Form Factors

We report results on the $\pi$-$\gamma$ transition form factor obtained within the hard scattering approach including transverse momentum effects and Sudakov corrections. The results clearly favor distribution amplitudes close to the asymptotic form, $\sim x_1x_2$, and disfavor distribution amplitudes which are strongly concentrated in the end-point regions. This observation is backed by information on the elastic form factor of the pion and on its valence quark distribution function. Applications of our approach to the $\eta$-$\gamma$ and $\eta^\prime$-$\gamma$ transition form factors are discussed as well. Combining the form factor data with the two-photon decay widths, we determine the $\eta$ and the $\eta^\prime$ decay constants and the $\eta$-$\eta^\prime$ mixing angle.