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

A new experiment studying radiative pion decay, ${\pi }^{+}\to e^{+}{\nu }_e\gamma$ 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 $\gamma$, the ratio of the axial-vector to vector weak pion form factors, of 0.25 ± 0.12. The new world average is $\gamma =0.39\mathrm{}\pm 0.06$.