A new search for the $K_{L} \to \pi^0 \nu \overline{\nu}$ and $K_{L} \to \pi^{0} X^{0}$ decays

TL;DR

This paper reports a search for rare CP-violating kaon decays using data from the J-PARC KOTO experiment, setting new upper limits on the branching fractions of these elusive processes.

Contribution

It presents the first experimental upper limits on the branching fractions for $K_{L} o ext{pi}^0 X^0$ decays, expanding the search for new physics beyond the Standard Model.

Findings

Observed one candidate event for $K_{L} o ext{pi}^0 u ar{ u}$ decay.

Set an upper limit of $5.1 imes 10^{-8}$ for the $K_{L} o ext{pi}^0 u ar{ u}$ branching fraction.

Established a new upper limit of $3.7 imes 10^{-8}$ for $K_{L} o ext{pi}^0 X^0$ decay.

Abstract

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to \pi^0 \nu \overline{\nu}$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to \pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$.