A puzzle of the pion-photon transition form factor - resolved?

TL;DR

This paper uses QCD sum rules to analyze pseudoscalar meson transition form factors, finding that most data align with pQCD predictions at high momentum transfer, with recent Belle results helping resolve previous discrepancies.

Contribution

It provides a unified analysis of transition form factors using QCD sum rules, clarifying the behavior across different mesons and momentum transfers, and addressing the BaBar puzzle.

Findings

Most transition form factors agree with pQCD saturation at high momentum transfer.

Light pseudoscalar mesons reach saturation at relatively small momentum transfer.

Belle's recent measurement aligns with theoretical expectations, resolving previous puzzles.

Abstract

By means of QCD sum rules in local-duality limit, we analyze the behaviour of the form factors for the transitions of a real and a virtual photon to some pseudoscalar meson as functions of the involved momentum transfer. Except for the findings of BaBar for the neutral-pion form factor, the experimental data for all these transition form factors are compatible with saturation for large momentum transfer predicted by pQCD factorization. For light pseudoscalar mesons, saturation is observed already at relatively small momentum transfer, whereas for the eta-c meson it sets in only at larger momentum transfer. A recent measurement of the neutral-pion transition form factor by Belle seems to resolve this disturbing puzzle as its outcome is compatible with both saturation for relatively small momentum transfer and the behaviour of the eta and eta' transition form factors at large momentum transfer.