CP Conserving and Violating Contributions to $K_L\to {\pi }^{+}{\pi }^{-}\nu \bar{\nu }$

We study both CP conserving and violating contributions to the decay $K_L\to {\pi }^{+}{\pi }^{-}\nu \bar{\nu }$. We find that the decay branching ratio is dominated by the CP conserving part. In the standard CKM model, we estimate that for $m_t\sim 174GeV$, the branching ratio due to the CP conserving (violating) contributions can be as large as $4.4\times {10}^{-13}$ ($1.0\times 10^{-14}$).