Rare Kaon Decays

Abstract

The experimental status of the $K\to \ddot{I}€\hat{I}½\bar{\hat{I}}½$ search is presented. The $K\to \ddot{I}€\hat{I}½\bar{\hat{I}}½$ decay is sensitive to New Physics because it is theoretically pristine and highly suppressed. The $K_L^0\to \ddot{I}{€}^0\hat{I}½\bar{\hat{I}}½$ search is performed by the KOTO experiment and a branching fraction limit of $\mathcal{B}(K_L^0\to \ddot{I}{€}^0\hat{I}½\bar{\hat{I}}½)$ $<$ 3.0 $\times$ 10${}^{-9}$ (90% confidence level) was set. This limit is $\mathcal{O}(100)$ times larger than the Standard Model prediction. The $K^{+}\to \ddot{I}{€}^{+}\hat{I}½\bar{\hat{I}}½$ search is performed by the NA62 experiment and $\mathcal{B}(K^{+}\to \ddot{I}{€}^{+}\hat{I}½\bar{\hat{I}}½)$ $=$ (10.6 ${}_{-3.5}^{+4.0}$ ${|}_{\text{stat.}}$) $\pm$ 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 $K\to \ddot{I}€X$ decay. A projection of the $K\to \ddot{I}€\hat{I}½\bar{\hat{I}}½$ search in the future is also given.