We report results from Experiment 871, performed at the BNL AGS, of a measurement of the branching ratio $K_L^0\to {\mu }^{+}{\mu }^{-}$ with respect to the $\mathrm{CP}$-violating mode $K_L^0\to {\pi }^{+}{\pi }^{-}$. This experiment detected over 6200 candidate ${\mu }^{+}{\mu }^{-}$ events, a factor of 6 more than that seen in all previous measurements combined. The resulting branching ratio $\Gamma (K_L^0\to {\mu }^{+}{\mu }^{-})/\Gamma (K_L^0\to {\pi }^{+}{\pi }^{-})=(3.474\pm 0.057)\times {10}^{-6\mathrm{}}$ leads to a branching fraction $B(K_L^0\to {\mu }^{+}{\mu }^{-})=(7.18\pm 0.17)\times {10}^{-9\mathrm{}}$, which is consistent with the current world average, and reduces the uncertainty in this decay mode by a factor of 3.

• Received 24 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.1389