Extracting the chiral anomaly from $e^{+}{e}^{-}\to 3\ddot{I}€$

Abstract

The strength of the interaction of three pions and a photon, $F_{3\ddot{I}€}$ is predicted by the axial anomaly in terms of the pion decay constant, a relation that is frequently used to constrain low-energy radiative processes involving pions, but only tested experimentally at the $10\%$ level. Here, we present a new avenue to test this prediction, via a fit of a dispersive description of the $γ^*\to3π$ amplitude to data for $e^{+}{e}^{-}\to 3\ddot{I}€$. From the global fit to SND, CMD-2, and BaBar data we obtain $F_{3\ddot{I}€}=33.1(1.7){\text{GeV}}^{-3}$, in agreement with the chiral prediction at the level of $5\%$. We also consider dispersive fits to the recent data by Belle II, in which case we observe tensions with the dispersive constraints, the width parameters of $\ddot{I}‰$ and $\ddot{I}•$, and the chiral anomaly.

17 pages, 1 figure; journal version