Search for $K_L \!\to\! \pi^0 \nu \overline{\nu}$ and $K_L \!\to\! \pi^0 X^0$ Decays at the J-PARC KOTO Experiment

A. S. Kurilin; B. Beckford; C. Lin; D. Mcfarland; D. Naito; E. J. Kim; G. Y. Lim; H. Haraguchi; H. Nanjo; H. Nishimiya

arxiv: 1810.09655 · v2 · pith:DXDHA7HGnew · submitted 2018-10-23 · ✦ hep-ex

Search for K_L \!to\! π⁰ ν overline{ν} and K_L \!to\! π⁰ X⁰ Decays at the J-PARC KOTO Experiment

KOTO Collaboration: J. K. Ahn , B. Beckford , J. Beechert , K. Bryant , M. Campbell , S. H. Chen , J. Comfort , K. Dona

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N. Hara H. Haraguchi Y. B. Hsiung M. Hutcheson T. Inagaki I. Kamiji N. Kawasaki E. J. Kim J. L. Kim Y. J. Kim J. W. Ko T. K. Komatsubara K. Kotera A. S. Kurilin J. W. Lee G. Y. Lim C. Lin Q. Lin Y. Luo J. Ma Y. Maeda T. Mari T. Masuda T. Matsumura D. Mcfarland N. McNeal J. Micallef K. Miyazaki R. Murayama D. Naito K. Nakagiri H. Nanjo H. Nishimiya T. Nomura M. Ohsugi H. Okuno M. Sasaki N. Sasao K. Sato T. Sato Y. Sato H. Schamis S. Seki N. Shimizu T. Shimogawa T. Shinkawa S. Shinohara K. Shiomi S. Su Y. Sugiyama S. Suzuki Y. Tajima M. Taylor M. Tecchio M. Togawa Y. C. Tung Y. W. Wah H. Watanabe J. K. Woo T. Yamanaka H. Y. Yoshida

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A search for the rare decay $K_L \!\to\! \pi^0 \nu \overline{\nu}$ was performed. With the data collected in 2015, corresponding to $2.2 \times 10^{19}$ protons on target, a single event sensitivity of $( 1.30 \pm 0.01_{\rm stat} \pm 0.14_{\rm syst} ) \times 10^{-9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0 \times 10^{-9}$ for the branching fraction of $K_L \!\to\! \pi^0 \nu \overline{\nu}$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for $K_L \!\to\! \pi^0 X^0$ was also set as $2.4 \times 10^{-9}$ at the 90% C.L., where $X^0$ is an invisible boson with a mass of $135~{\rm MeV}/c^2$.

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