The $\pi\gamma$ transition form factor and its impact on charmonium decays into two pions

Abstract

The analysis of the $\pi\gamma$ transition form factor provides severe constraints on the pion's wave function. This information is used to examine charmonium decays into two pions critically. It will be argued that the standard perturbative QCD analysis of these reactions fails, i.e. the need for additional contributions can convincingly be demonstrated. Colour-octet admixtures to the charmonium states are proposed as a possible dynamical mechanisms to solve the puzzle. Consequences of the $\pi\gamma$ analysis for the electromagnetic form factor of the pion are also discussed.