Study of the $K_L\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\pi }^0\nu \bar{\nu }$ Decay at the J-PARC KOTO Experiment

The rare decay $K_L\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\pi }^0\nu \bar{\nu }$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $( 7.20 \pm 0.05_{\rm stat} \pm 0.66_{\rm syst} ) \times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{\pm }$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22\pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9\times {10}^{-9}$ on the branching fraction of $K_L !\to! \pi^0 \nu \overline{\nu}$ at the 90% confidence level.