Probing the Standard Model via rare pion and muon decays

TL;DR

This paper reports precise measurements of rare pion and muon decays using a CsI calorimeter, confirming Standard Model predictions and improving the accuracy of decay parameters and branching ratios.

Contribution

It provides the most accurate measurements of rare pion and muon decay parameters, enhancing the precision of decay branching ratios and form factors compared to previous data.

Findings

Measured pi+ -> pi0 e+ nu branching ratio with 0.55% uncertainty

Improved precision of weak axial and vector form factors F_A and F_V

Muon decay distributions consistent with Standard Model predictions

Abstract

The PIBETA collaboration has used a non-magnetic pure CsI calorimeter operating at the Paul Scherrer Institute to collect the world's largest sample of rare pion and muon decays. We have extracted the absolute pi+ -> pi0 e+ nu decay branching ratio with the 0.55 % total uncertainty. The pi+ -> e + nu gamma data set was used to extract weak axial and vector form factors F_A and F_V, yielding a significant improvement in the precision of F_A and F_V. The mu+ -> e+ nu nu gamma distributions were well described with the two-parameter (rho_SM,eta_bar=0) solution. These results bring major improvements in accuracy over the current Particle Data Group listings and agree well with the predictions of the Standard Model.