Novel insights into the $\gamma\gamma^*\to \pi^0$ transition form factor

TL;DR

This paper investigates deviations in the pion transition form factor at high momentum transfers, suggesting that non-factorizable contributions from intermediate hadron loops may explain the observed discrepancies from perturbative QCD predictions.

Contribution

It introduces a novel approach linking non-factorizable contributions to intermediate hadron loops, challenging the dominance of collinear factorization at high Q^2 for the pion TFF.

Findings

Non-factorizable contributions significantly affect the pion TFF at high Q^2.

Intermediate hadron loops can account for deviations from pQCD expectations.

The approach provides a new perspective on the fundamental understanding of the pion structure.

Abstract

BaBar's observation of significant deviations of the pion transition form factor (TFF) from the asymptotic expectation with $Q^2>9$ GeV$^2$ has brought a serious crisis to a fundamental picture established for such a simplest $q\bar{q}$ system by perturbative QCD, i.e. the dominance of collinear factorization at high momentum transfers for the pion TFF. We show that non-factorizable contributions due to open flavors in $\gamma\gamma^*\to\pi^0$ could be an important source that contaminates the pQCD asymptotic limit and causes such deviations with $Q^2>9$ GeV$^2$. Within an effective Lagrangian approach, the non-factorizable amplitudes can be related to intermediate hadron loops, i.e. $K^{(*)}$ and $D^{(*)}$ etc, and their corrections to the $\pi^0$ and $\eta$ TFFs can be estimated.