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

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 $(7.1\pm 0.3_{stat.}\pm 1.6_{syst.})\times 10^{-8}$, 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.