– 05 KEK Preprint 99-183 Measurement of Structure Dependent K + → μ + ν μ γ Decay

We report the first measurement of a structure dependent component in the decay K→μνμγ. Using the kinematic region where the muon kinetic energy is greater than 137 MeV and the photon energy is greater than 90 MeV, we find that the absolute value of the sum of the vector and axialvector form factors is |FV + FA| = 0.165± 0.007± 0.011. This corresponds to a branching ratio of BR(SD) = (1.33 ± 0.12± 0.18)× 10−5. We also set the limit −0.04 < FV − FA < 0.24 at 90% c.l. Typeset using REVTEX