Observation of $\mathit{CP}$ Violation in $K_L\to {\pi }^{+}{\pi }^{-}{\mathit{e}}^{+}{\mathit{e}}^{-}$ Decays

We report the first observation of a manifestly $\mathrm{CP}$ violating effect in the $K_L\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ decay mode. A large asymmetry was observed in the distribution of these decays in the $\mathrm{CP}$-odd and $T$-odd angle $\phi$ between the decay planes of the $e^{+}{e}^{-}$ and ${\pi }^{+}{\pi }^{-}$ pairs in the $K_L$ center of mass system. After acceptance corrections, the overall asymmetry is found to be $[13.6\pm 2.5(\mathrm{stat})\pm 1.2(\mathrm{syst}\left)\right]\%$. This is the largest $\mathrm{CP}$-violating effect yet observed when integrating over the entire phase space of a mode and the first such effect observed in an angular variable.