Pion Distribution Amplitude and Photon-to-Pion Transition Form Factor in QCD

TL;DR

This paper reviews the current understanding of the pion distribution amplitude in QCD, analyzing theoretical models and experimental data, including recent lattice and BaBar results, to assess their consistency and implications.

Contribution

It provides a comprehensive analysis of the pion distribution amplitude using QCD sum rules, light cone sum rules, lattice QCD, and experimental data, highlighting challenges posed by recent BaBar results.

Findings

Lattice QCD data supports certain pion DA models.

BaBar data challenges existing QCD predictions.

Light cone sum rules effectively analyze transition form factors.

Abstract

We discuss the status of the pion distribution amplitude (DA) in connection with QCD sum rules and experimental data on the $\gamma^*\gamma^\to \pi^0$ transition form factor. Contents: (a) Pion DA in generalized QCD Sum Rules (SRs); (b) Light Cone Sum Rules (LCSR) analysis of the CLEO data for the $\gamma^*\gamma\to\pi^{0}$ transition form factor; (c) Recent lattice QCD data for the pion DA; (d) BaBar data---a challenge for QCD?