Observation of the DecayKL0→μ+μ−

An experiment performed at the Brookhaven National Laboratory alternating-gradient synchrotron has yielded six events above negligible background which satisfy criteria for the decay $K_L^{}{}_{}{}^0\to {\mu }^{+}{\mu }^{-}$. The $K_L^{}{}_{}{}^0$ flux, measured by means of the decay $K_L^{}{}_{}{}^0\to {\pi }^{+}{\pi }^{-}$, leads to a value for the branching ratio $\frac{\Gamma (K_L^{}{}_{}{}^0\to {\mu }^{+}{\mu }^{-})}{\Gamma (K_L\to \mathrm{all})}=11\mathrm{}\times {10}^{-9\mathrm{}}$.