Rare Kaon Decays

The experimental status of the $K \to \pi \nu \bar{\nu}$ search is presented. The $K \to \pi \nu \bar{\nu}$ decay is sensitive to New Physics because it is theoretically pristine and highly suppressed. The $K_L^0 \to \pi^0 \nu \bar{\nu}$ search is performed by the KOTO experiment and a branching fraction limit of $\mathcal{B}(K_L^0 \to \pi^0 \nu \bar{\nu})$ $<$ 3.0 $\times$ 10$^{-9}$ (90% confidence level) was set. This limit is $\mathcal{O}(100)$ times larger than the Standard Model prediction. The $K^+ \to \pi^+ \nu \bar{\nu}$ search is performed by the NA62 experiment and $\mathcal{B}(K^+ \to \pi^+ \nu \bar{\nu})$ $=$ (10.6 $^{+4.0}_{-3.5}$ $|_{\text{stat.}}$) $\pm$ 0.9$_{\text{syst.}}$) $\times$ 10$^{-11}$ (68% confidence level) was set. This shows an agreement with the Standard Model. Both measurements can also be used to search for the dark particle $X$ via the $K \to \pi X$ decay. A projection of the $K \to \pi \nu \bar{\nu}$ search in the future is also given.