The pion transition form factor and the pion distribution amplitude

Abstract.Recent BaBar data on the pion transition form factor, whose Q2-dependence is much steeper then predicted by asymptotic Quantum Chromodynamics (QCD), have caused a renewed interest in its theoretical description. We present here a formalism based on a model-independent description for low photon virtuality and a high photon virtuality description based on QCD, which match at a scale Q0. The high photon virtuality description incorporates a flat pion distribution amplitude, $ \phi$(x) = 1 , at the matching scale Q0 and QCD evolution from Q0 to Q > Q0 . The flat pion distribution is connected, through soft pion theorems and chiral symmetry, to the pion valence parton distribution at the same low scale Q0. The procedure leads to a good description of the data, and by incorporating additional twist-three effects, to an excellent description of the data.