Measurement of Structure Dependent K^+ -> mu^+ nu gamma

TL;DR

This paper presents the first measurement of the structure-dependent component in the decay K^+ -> mu^+ nu gamma, determining form factors and branching ratios with precise experimental data.

Contribution

It provides the first experimental determination of the vector and axial-vector form factors in the K^+ -> mu^+ nu gamma decay.

Findings

|F_V+F_A| = 0.165  0.007  0.011

Branching ratio BR(SD^+) = (1.33  0.12  0.18) imes 10^{-5}

Limit -0.04 < F_V-F_A < 0.24 at 90% confidence level

Abstract

We report the first measurement of a structure dependent component in the decay K^+ -> mu^+ nu gamma. 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 axial-vector form factors is |F_V+F_A| =0.165 \pm 0.007 \pm 0.011. This corresponds to a branching ratio of BR(SD^+) = (1.33 \pm 0.12 \pm 0.18) \times 10^{-5}. We also set the limit -0.04 < F_V-F_A < 0.24 at 90% c.l.