Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay

TL;DR

This study precisely measures the pion's axial form factor through radiative decay analysis, providing new experimental data that suggest deviations from Standard Model predictions in specific kinematic regions.

Contribution

The paper presents the first detailed measurement of the pion axial form factor using Dalitz distributions from a large event sample, revealing potential deviations from theoretical expectations.

Findings

Measured F_A/F_V ratio as 0.443(15)

Identified deviations in high-E(gamma)/low-E(e+) region

Provided absolute branching ratios in three kinematic regions

Abstract

We have studied radiative pion decays Pi+ -> e+ nu gamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 42,209 events we have evaluated absolute Pi -> e nu gamma branching ratios in the three regions. Minimum chi^2 fits to the integral and differential (E(e+),E(gamma)) distributions result in the axial-to-vector weak form factor ratio of gamma = F_A/F_V = 0.443(15),or F_A = 0.0115(4) with F_V = 0.0259. However, deviations from Standard Model predictions in the high-E(gamma)/low-E(e+) kinematic region indicate the need for further theoretical and experimental work.