Chiral Anomaly Effects and the BaBar Measurements of the $\gamma\gamma^{*}\to \pi^{0}$ Transition Form Factor

TL;DR

This paper explains the BaBar measurements of the $ o ext{pi}^0$ transition form factor using anomaly effects, showing a specific Q^2 dependence that matches experimental data and revises previous theoretical understanding.

Contribution

The paper introduces a novel Q^2 dependence for the transition form factor based on anomaly cancellations, aligning theory with recent BaBar experimental results.

Findings

The form factor $F(Q^2)$ behaves as $(m^2/Q^2)( ext{ln}(Q^2/m^2))^2$.

The model reproduces BaBar data for $Q^2F(Q^2)$ with $m=132$ MeV.

The approach also fits lower Q^2 CLEO data.

Abstract

The recent BaBar measurements of the $\gamma\gamma^{*}\to \pi^{0}$ transition form factor show spectacular deviation from perturbative QCD prediction for large space-like $Q^{2}$ up to $34\,\rm GeV^{2}$. When plotted against $Q^{2}$, $Q^{2}F(Q^{2})$ shows steady increase with $Q^{2}$ in contrast with the flat $Q^{2}$ behavior predicted by perturbative QCD, and at $34\,\rm GeV^{2}$ is more than 50% larger than the QCD prediction. Stimulated by the BaBar measurements, we revisit our previous paper on the cancellation of anomaly effects in high energy processes $Z^{0}\to \pi^{0}\gamma$, $e^{+}e^{-}\to \pi^{0}\gamma$ and apply our results to the $\gamma^{*}\gamma\to \pi^{0}$ transition form factor measured in the $e^{+}e^{-}\to e^{+}e^{-}\pi^{0}$ process with one highly virtual photon. We find that, the transition form factor $F(Q^{2})$ behaves as $(\frac{m^{2}}{Q^{2}})\times (\ln(Q^{2}/m^{2}))^{2}$ and produces a striking agreement with the BaBar data for $Q^{2}F(Q^{2})$ with $m=132\,\rm MeV$ which also reproduces very well the CLEO data at lower $Q^{2}$.