Status and future prospects for rare kaon decay measurements at KOTO and KOTO II

The KOTO experiment at J-PARC searches for the rare decay of the neutral kaon, $K_{L}\rightarrow\pi^{0}\nu\bar{\nu}$. The $K_{L}\rightarrow\pi^{0}\nu\bar{\nu}$ decay in the standard model is highly suppressed, and its branching ratio is predicted to be $3\times10^{-11}$ with a small theoretical uncertainty. The analysis of the $K_{L}\rightarrow\pi^{0}\nu\bar{\nu}$ search based on data collected in 2021 set the most stringent upper limit on the branching ratio as $2.2\times10^{-9}$ at the 90% confidence level. The KOTO experiment has further accumulated data since 2023 with upgraded hardware and data acquisition systems. With several years of data collection in the future, we aim to achieve a single event sensitivity better than $1\times10^{-10}$. A next-generation experiment, KOTO II, is also under discussion toward measurements of the branching ratio of $K_{L}\rightarrow\pi^{0}\nu\bar{\nu}$. In this report, the latest status of the analysis for the $K_{L}\rightarrow\pi^{0}\nu\bar{\nu}$ search at KOTO and a current design for the KOTO II experiment will be described.