Experimental Limits on the Decays ${K_L}^0\to {\mu }^{+}{\mu }^{-},e^{+}{e}^{-}, \mathrm{and} {\mu }^{\pm }{e}^{\mp }$

We have performed a search at the Bevatron for the decays ${K_L}^0\to {\mu }^{+}{\mu }^{-},e^{+}{e}^{-}, \mathrm{and} {\mu }^{\pm }{e}^{\mp }$ with a double magnetic spectrometer using wire spark chambers. Over ${10}^6$ observed ${K_L}^0\to {\pi }^{+}{\pi }^{-}$ decays determine the normalization for the di-lepton decay modes. No $e^{+}{e}^{-}$ or ${\mu }^{\pm }{e}^{\mp }$ events were observed. For each of these decays the upper limit on the branching ratio relative to all modes is 1.57 × ${10}^{-9\mathrm{}}$ (90% confidence level). For the decay ${K_L}^0\to {\mu }^{+}{\mu }^{-}$, the limit is 1.82 × ${10}^{-9\mathrm{}}$ (90% confidence level).