The pion transition form factor and the pion distribution amplitude

TL;DR

This paper develops a combined low and high energy formalism to explain the BaBaR data on the pion transition form factor, emphasizing a flat pion distribution amplitude and QCD evolution, achieving good to excellent data agreement.

Contribution

It introduces a model-independent low energy and QCD-based high energy matching framework with a flat pion distribution amplitude, improving data description.

Findings

Good description of BaBaR data achieved

Inclusion of twist three effects enhances fit quality

Connects pion distribution to valence parton distribution at low scale

Abstract

Recent BaBaR data on the pion transition form factor, whose Q^2 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 low energy description and a high energy description based on QCD, which match at a scale Q_0. The high energy description incorporates a flat pion distribution amplitude, phi(x)=1, at the matching scale Q_0 and QCD evolution from Q_0 to Q>Q_0. The flat pion distribution is connected, through soft pion theorems and chiral symmetry, to the pion valance parton distribution at the same low scale Q_0. The procedure leads to a good description of the data, and incorporating additional twist three effects, to an excellent description of the data.