Search for $K_L \!\to\! \pi^0 \nu \overline{\nu}$ and $K_L \!\to\! \pi^0 X^0$ Decays at the J-PARC KOTO Experiment

TL;DR

This paper reports a search for rare $K_L$ decays at J-PARC KOTO, setting new upper limits on their branching fractions, significantly improving previous constraints and exploring potential new physics involving invisible particles.

Contribution

First search at J-PARC KOTO for $K_L o ext{invisible}$ decays, establishing the most stringent upper limits to date on these rare processes.

Findings

No candidate events observed for $K_L o ext{invisible}$ decays.

Set upper limit of $3.0 imes 10^{-9}$ on $K_L o ext{pi}^0 u ar{ u}$.

Set upper limit of $2.4 imes 10^{-9}$ on $K_L o ext{pi}^0 X^0$.

Abstract

A search for the rare decay $K_L \!\to\! \pi^0 \nu \overline{\nu}$ was performed. With the data collected in 2015, corresponding to $2.2 \times 10^{19}$ protons on target, a single event sensitivity of $( 1.30 \pm 0.01_{\rm stat} \pm 0.14_{\rm syst} ) \times 10^{-9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0 \times 10^{-9}$ for the branching fraction of $K_L \!\to\! \pi^0 \nu \overline{\nu}$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for $K_L \!\to\! \pi^0 X^0$ was also set as $2.4 \times 10^{-9}$ at the 90% C.L., where $X^0$ is an invisible boson with a mass of $135~{\rm MeV}/c^2$.