First Search for the $K_L \to \pi^0 \gamma$ Decay

TL;DR

This paper reports the first experimental search for the forbidden decay $K_L o au^0 o au^0 o au^0$, setting an upper limit on its branching fraction, and thus testing Lorentz invariance.

Contribution

It presents the first search for the Lorentz-invariance-violating decay $K_L o au^0$, establishing an experimental upper limit and demonstrating the sensitivity of the J-PARC KOTO experiment.

Findings

No candidate events observed in the signal region.

Upper limit on the branching fraction set at $1.7 imes 10^{-7}$.

Single event sensitivity of $(7.1 imes 10^{-8})$.

Abstract

We report the first search for the $K_L \to \pi^0 \gamma$ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of $(7.1\pm 0.3_{\rm stat.} \pm 1.6_{\rm syst.})\times 10^{-8}$, no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be $1.7\times 10^{-7}$ at the 90\% confidence level.