Search for the $K_L\to {\pi }^0\nu \bar{\nu }$ Decay at the J-PARC KOTO Experiment

We performed a search for the $K_L\to {\pi }^0\nu \bar{\nu }$ decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to $0.252\pm {0.055}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}$${}_{-0.067}^{+0.052}$${}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}}$. With a single event sensitivity of $(9.33\pm {0.06}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.84}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}})\times {10}^{-10}$, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be $2.2\times {10}^{-9}$ at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for $K_L\to {\pi }^0{X}^0$ was also performed, where $X^0$ is an invisible boson with a mass ranging from 1 MeV/$c^2$ to 260 MeV/$c^2$. For $X^0$ with a mass of 135 MeV/$c^2$, an upper limit on the branching fraction of $K_L\to {\pi }^0{X}^0$ was set to be $1.6\times {10}^{-9}$ at the 90% C.L.