A Study of the Structure of the Pi Meson via the Radioactive Pion Decay

Two of the most important parameters describing pi meson structure are F V and F A , the vector and axial vector form factors, respectively. Their values can be determined primarily by measuring the π + → e + νγ radiative pion decay. This study is based on the 20,182 radiative pion decay events collected at the Paul Scherrer meson factory using a large acceptance detector and a stopped pion beam. A detailed analysis of the dependence of the decay rate on kinematic parameters has resulted in the following best fit values: F V =0.0262(15), F V =0.0118(3) and a=0.241(93), where the parameter a describes the momentum dependence of the form factors. This is the first experimental determination of a ever. The reported values of F V and F A represent nearly sixand twofold improvements in accuracy over previously published results, respectively, and are in an excellent agreement with predictions of the Standard Model.

Note: Abstract extracted from PDF text