Search for the $K_L\to \ddot{I}{€}^0\hat{I}½\bar{\hat{I}}½$ Decay at the J-PARC KOTO Experiment

Abstract

We performed a search for the $K_L\to \ddot{I}{€}^0\hat{I}½\bar{\hat{I}}½$ 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{stat}}$${}_{-0.067}^{+0.052}$${}_{\mathrm{syst}}$. With a single event sensitivity of $(9.33\pm 0.06_{\mathrm{stat}}\pm 0.84_{\mathrm{syst}})\times 10^{-10}$, 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 $K_L\to \ddot{I}{€}^0{X}^0$ 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 $K_L\to \ddot{I}{€}^0{X}^0$ was set to be $1.6\times 10^{-9}$ at the 90% C.L.

6 pages, 4 figures; published version, the last sentence in the section of "Conclusion and prospects" was modified