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

TL;DR

This paper reports a search for the rare decay $K_L o c0 bc bc$ at J-PARC, setting new upper limits on its branching ratio and exploring potential invisible boson emissions, with improved background suppression and analysis methods.

Contribution

The study introduces new detectors and analysis techniques that significantly improve background suppression and sensitivity in searching for rare $K_L$ decay modes.

Findings

No events observed in the $K_L o c0 bc bc$ signal region.

Upper limit on $K_L o c0 bc bc$ branching ratio set at $2.2 imes 10^{-9}$.

Upper limit on $K_L o c0 X^0$ with $X^0$ mass 135 MeV/$c^2$ is $1.6 imes 10^{-9}$.

Abstract

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\pm0.055_{\mathrm{stat}}$$^{+0.052}_{-0.067}$$_{\mathrm{syst}}$. With a single event sensitivity of $(9.33 \pm 0.06_{\rm stat} \pm 0.84_{\rm syst})\times 10^{-10}$, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be $2.2\times10^{-9}$ 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 $1.6\times10^{-9}$ at the 90% C.L.