Search for <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> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>K</mml:mi><mml:mi>L</mml:mi></mml:msub><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup><mml:msup><mml:mi>X</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math> Decays at the J-PARC KOTO Experiment

Abstract

A search for the rare decay ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ was performed. With the data collected in 2015, corresponding to $2.2\ifmmode\times\else\texttimes\fi{}{10}^{19}$ protons on target, a single event sensitivity of $(1.30\ifmmode\pm\else\textpm\fi{}0.0{1}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.1{4}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for the branching fraction of ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{X}^{0}$ was also set as $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ at the 90% C.L., where ${X}^{0}$ is an invisible boson with a mass of $135\text{ }\text{ }\mathrm{MeV}/{c}^{2}$.