First Search for the $K_L \to π^0 γ$ Decay

J. K. Ahn, B. Beckford, M. Campbell, S. H. Chen, J. M. Choi, J. Comfort, K. Dona, M. S. Farrington, N. Hara, H. Haraguchi, Y. B. Hsiung, M. Hutcheson, T. Inagaki, M. Isoe, I. Kamiji, E. J. Kim, J. L. Kim, H. M. Kim, T. K. Komatsubara, K. Kotera, J. W. Lee, G. Y. Lim, C. Lin, Q. S. Lin, Y. Luo, T. Mari, T. Matsumura, D. Mcfarland, K. Miyazaki, R. Murayama, K. Nakagiri, H. Nanjo, H. Nishimiya, Y. Noichi, T. Nomura, T. Nunes, M. Ohsugi, H. Okuno, J. C. Redeker, K. Sato, T. Sato, Y. Sato, N. Shimizu, T. Shimogawa, T. Shinkawa, S. Shinohara, K. Shiomi, R. Shiraishi, S. Su, Y. Sugiyama, S. Suzuki, Y. Tajima, M. Taylor, M. Tecchio, M. Togawa, T. Toyoda, Y. C. Tung, Q. H. Vuong, Y. W. Wah, H. Watanabe, T. Yamanaka, H. Y. Yoshida

Published 2020-06-26Version 1

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.