Can we understand an auxetic pion-photon transition form factor within QCD? BaBar faces Belle

TL;DR

This paper presents an advanced QCD-based analysis of the pion-photon transition form factor, incorporating photon virtuality effects, and compares theoretical predictions with experimental data, highlighting agreements and conflicts with recent measurements.

Contribution

It introduces an improved light-cone sum rule approach that accounts for photon virtuality and employs multiple Gegenbauer coefficients, providing a more accurate theoretical description of the form factor.

Findings

Theoretical predictions align with Belle data supporting scaling behavior.

The model reproduces /' data from CLEO and BaBar.

Strong conflict with BaBar's auxetic trend above 10 GeV^2.

Abstract

A state-of-the-art analysis of the pion-photon transition form factor is presented based on an improved theoretical calculation that includes the effect of a finite virtuality of the quasi-real photon in the method of light-cone sum rules. We carry out a detailed statistical analysis of the existing experimental data using this method and by employing pion distribution amplitudes with up to three Gegenbauer coefficients $a_2, a_4, a_6$. Allowing for an error range in the coefficient $a_6\approx 0$, the theoretical predictions for $\gamma^*\gamma\to\pi^0$ obtained with nonlocal QCD sum rules are found to be in good agreement with all data that support a scaling behavior of the transition form factor at higher $Q^2$, like those of the Belle Collaboration. The data on $\gamma^*\gamma\to\eta/\eta'$ from CLEO and \babar are also reproduced, while there is a strong conflict with the auxetic trend of the \babar data above 10 GeV$^2$. The broader implications of these findings are discussed.