Search for the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">→</mml:mo> <mml:msup> <mml:mrow> <mml:mi>π</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover accent="true"> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> Decay at the J-PARC KOTO Experiment
J. K. Ahn, Michael Steven Farrington, Mario González, N. Grethen, Kouzou Hanai, Naoya Hara, Hiroki Haraguchi, Y. Hsiung, T. Inagaki, M. Katayama, Takayuki Kato, Y. Kawata, E. J. Kim, H. M. Kim, A. Kitagawa, T. K. Komatsubara, K. Kotera, S. K. Lee, Xinlin Li, G. Y. Lim, C. Lin, Y. Luo, T. Mari, Takeshi Matsumura, Ichiro Morioka, H. Nanjo, Han Nishimiya, Y. Noichi, T. Nomura, Kisho Ono, M. Osugi, P. Paschos, Joseph Redeker, T. Sato, Y. Sato, T. Shibata, N. Shimizu, T. Shinkawa, K. Shiomi, Ryutaro Shiraishi, S. Suzuki, Y. Tajima, N. Taylor, Y. C. Tung, Y. W. Wah, H. Watanabe, Tong Wu, T. Yamanaka, H. Y. Yoshida
Abstract
We performed a search for the K_{L}→π^{0}νν[over ¯] decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to 0.252±0.055_{stat} _{-0.067^{syst}}^{+0.052}. With a single event sensitivity of (9.33±0.06_{stat}±0.84_{syst})×10^{-10}, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be 2.2×10^{-9} at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for K_{L}→π^{0}X^{0} was also performed, where X^{0} is an invisible boson with a mass ranging from 1 to 260 MeV/c^{2}. For X^{0} with a mass of 135 MeV/c^{2}, an upper limit on the branching fraction of K_{L}→π^{0}X^{0} was set to be 1.6×10^{-9} at the 90% C.L.