Study of the $K_L \!\to\! \pi^0 \nu \overline{\nu}$ Decay at the J-PARC KOTO Experiment

TL;DR

This study reports on the search for the rare decay $K_L o \pi^0 u ar{ u}$ at J-PARC KOTO, observing three candidate events and setting an upper limit on its branching fraction, consistent with background expectations.

Contribution

First experimental search for $K_L o \pi^0 u ar{ u}$ at J-PARC KOTO with new data, establishing an upper limit on the decay's branching fraction.

Findings

Observed three candidate events.

Estimated background of 1.22 ± 0.26 events.

Set an upper limit of 4.9 × 10^{-9} on the branching fraction.

Abstract

The rare decay $K_L \!\to\! \pi^0 \nu \overline{\nu}$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $( 7.20 \pm 0.05_{\rm stat} \pm 0.66_{\rm syst} ) \times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{\pm}$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22 \pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9 \times 10^{-9}$ on the branching fraction of $K_L \!\to\! \pi^0 \nu \overline{\nu}$ at the 90% confidence level.