A New Measurement of the Rare Decay $K^+\to π^+ μ^+ μ^-$

More than 400 $K^{+}\to\pi^{+}\mu^+\mu^-$ events were observed in a rare $K^+$ decay experiment at the AGS. Normalized to the $K^{+}\to\pi^{+}\pi^+\pi^-$ decay, the branching ratio is determined to be $(9.22 \pm 0.60 (stat) \pm 0.49 (syst))\times 10^{-8}$. This branching ratio and the $\mu\mu$ mass spectrum is in very good agreement with the measurement of the $K^{+}\to\pi^{+}e^+e^-$ decay, but deviates significantly from the previous measurement.