The photon-pion transition form factor for virtual photons

Abstract

We discuss the photon to meson transition form factor for virtual photons, which can be measured in e+ e- collisions. We demonstrate that this form factor is independent of the shape of the meson distribution amplitude over a wide kinematical range. This leads to a parameter-free prediction of perturbative QCD to leading twist accuracy, which has a status comparable to the famous leading-twist prediction of the cross section ratio R.

Comments: 4 pages, 5 figures. Talk delivered by C. Vogt at the workshop "e+ e- Physics at Intermediate Energies", SLAC, April 30 - May 2, 2001. v2: Some clarifications in the text