Search for the Rare DecaysKL→π0π0μ+μ−andKL→π0π0X0→π0π0μ+μ−

The KTeV E799 experiment has conducted a search for the rare decays, ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{X}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, where the ${X}^{0}$ is a possible new neutral boson that was reported by the HyperCP experiment with a mass of $(214.3\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$. We find no evidence for either decay. We obtain upper limits of $\mathrm{Br}({K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{X}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})l1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ and $\mathrm{Br}({K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})l9.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ at the 90% confidence level. This result rules out the pseudoscalar ${X}^{0}$ as an explanation of the HyperCP result under the scenario that the $\overline{d}s{X}^{0}$ coupling is completely real.