Precise measurement of the pion axial form factor in the π+→e+νγ decay -: art. no. 181804

We have studied radiative pion decays pi(+)-->e(+)nugamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 41 601 events we have evaluated absolute pi --> enugamma branching ratios in the three regions. Minimum chi(2) fits to the integral and differential (Ee+,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-Ee+ kinematic region indicate the need for further theoretical and experimental work.