Unique Determination of the Form-Factor Ratio in Radiative Pion Decay

Abstract

A new experiment studying radiative pion decay, ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}\ensuremath{\gamma}$ has been performed with the Crystal Box detector at the Clinton P. Anderson Meson Physics Facility (LAMPF). From a maximum-likelihood analysis of the Dalitz-plot distribution of 2364 coincident and random events, we obtain a unique value for $\ensuremath{\gamma}$, the ratio of the axial-vector to vector weak pion form factors, of 0.25 \ifmmode\pm\else\textpm\fi{} 0.12. The new world average is $\ensuremath{\gamma}=0.39\ifmmode\pm\else\textpm\fi{}0.06$.