| Abstract Two of the most important parameters describing pi meson structure are FV and FA , the vector and axial vector form factors; respectively. Their values can be determined primarily by measuring the pi+ → e+nugamma 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: FV=0.0262(15), FV=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 FV and FA represent nearly six- and twofold improvements in accuracy over previously published results, respectively, and are in an excellent agreement with predictions of the Standard Model. |
