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

Abstract

More than 400 $K^{+}\toπ^{+}μ^+μ^-$ events were observed in a rare $K^+$ decay experiment at the AGS. Normalized to the $K^{+}\toπ^{+}π^+π^-$ 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 $μμ$ mass spectrum is in very good agreement with the measurement of the $K^{+}\toπ^{+}e^+e^-$ decay, but deviates significantly from the previous measurement.

4 pages, 6 figures in 7 eps files. Paper to be submitted to Phys Rev Lett