Estimation of the branching ratio of the decayKL0→e+e−e+e−from theKL0→e+e−Xprocess

In an experiment to search for the decay $K_L^0\to \mathrm{ee}$ using the 12-GeV Proton Synchrotron at KEK, we observed 18 events with the $e^{+}{e}^{-}$ effective mass greater than 470 MeV/${\mathit{c}}^2$. They are presumed to come mainly from the decay $K_L^0\to e^{+}{e}^{-}{e}^{+}{e}^{-}$. Subtracting the background from the decay $K_L^0\to \mathrm{ee}\gamma$ and the external conversion of $\gamma$ in the decays $K_L^0\to \gamma \gamma$ and $K_L^0\to \mathrm{ee}\gamma$, the branching ratio of the decay $K_L^0\to e^{+}{e}^{-}{e}^{+}{e}^{-}$ is estimated to be (6±2±1)×${10}^{-8\mathrm{}}$. It is consistent with the previous experimental values.