A new evaluation of the pion weak form factors

TL;DR

This paper reports a precise measurement of pion radiative decay, revealing deviations from Standard Model predictions that suggest possible new physics, such as a pion tensor form factor.

Contribution

The study provides a new measurement of the pion axial-vector form factor and introduces evidence for a pion tensor form factor, challenging existing theoretical models.

Findings

Measured pion axial-vector form factor as 0.429(14)

Observed deviations from Standard Model predictions

Suggested presence of a pion tensor form factor

Abstract

We have used the PIBETA large acceptance detector for a precise measurement of the $\pi^+ \to e^+\nu\gamma$ radiative decay at rest, with broad phase space coverage. Using the CVC value for the pion vector form factor, $F_V = 0.0259(5)$, we have obtained a new value of the pion axial-vector form factor $\gamma = F_A/F_V = 0.429(14)$. However, significant deviations from the SM predictions are evident in our data. The discrepancy can be accounted for by introducing a pion tensor form factor of $F_T = -0.0016(3)$. The question of the true nature of the observed deviations remains open pending further theoretical and experimental study.