Study of theKL0→π0π0νν¯decay

The rare decay ${K}_{L}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on $9.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ ${K}_{L}^{0}$ decays, an upper limit of $8.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ was obtained for the branching fraction at 90% confidence level. We also set a limit on the ${K}_{L}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}X$ ($X\ensuremath{\rightarrow}\mathrm{\text{invisible particles}}$) process; the limit on the branching fraction varied from $7.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ to $4.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for the mass of $X$ ranging from $50\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ to $200\text{ }\text{ }\mathrm{MeV}/{c}^{2}$.