A measurement of Kevγ+ decay

The decay K⁺ → e⁺vγ has been observed. In a counter experiment at CERN, 56 events of this type have been identified by detection of a γ with an energy > 100 MeV and of an e⁺ with an energy between 236 MeV and the maximum e⁺ energy, 247 MeV. The angle between γ and e⁺ was > 120°. Thus, the experiment was sensitive only to the structure decay (SD) term proportional to the squared sum of vector- and axialvector amplitudes, |ν_K + a_K|², corresponding to the emission of right handed γ. We find Δ₊(SD)/Δ(K_e2) = 1.05_?0.30^+0.25 and Δ_(SD) < 85 (90% CL). Δ₊ is in agreement with theoretical predictions.