Experimental Measurement ofKL0→μ+μ−

Using a spark-chamber magnet spectrometer and applying very stringent requirements to eliminate background contamination, we find three events of the rare process ${K_L}^0\to {\mu }^{+}{\mu }^{-}$ corresponding to a branching ratio relative to ${K_L}^0\to {\pi }^{+}{\pi }^{-}$ of ${4.2}_{-2.6\mathrm{}}^{+5.1\mathrm{}}$×${10}^{-6\mathrm{}}$. Using the branching ratio $\frac{({K_L}^0\to {\pi }^{+}{\pi }^{-})}{({K_L}^0\to \mathrm{all})}=0.21\%$, we calculate the branching ratio to be ${8.8}_{-5.5\mathrm{}}^{+10.7\mathrm{}}$×${10}^{-9\mathrm{}}$ (90% confidence level) for the ${K_L}^0\to {\mu }^{+}{\mu }^{-}$ decay.