Measurement of the Branching Ratio and Form Factor of KL→μ+μ-γ

We report on the analysis of the rare decay KL→μ+μ-γ the 1997 data from the KTeV experiment at Fermilab. A total of 9327 candidate events are observed with 2.4% background, representing a factor of 40 increase in statistics over the current world sample. We find that B(KL→μ+μ-γ)=(3.62±0.04stat±0.08syst)×10-7. The form factor parameter αK* is measured to be αK*=-0.160-0.028+0.026. In addition, we make the first measurement of the parameter α from the D’Ambrosio-Isidori-Portolés form factor, finding α=-1.54±0.10. In that model, this α measurement limits the Cabibbo-Kobayashi-Maskawa parameter ρ>-0.2.