Search for a light pseudoscalar particle in the decay $K^0_L \to \pi^0   \pi^0 X$
  We performed a search for a light pseudoscalar particle $X$ in the decay $K_L^0->pi0pi0X$, $X->\gamma\gamma$ with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/$c^2$ was suggested by the HyperCP experiment. We found no evidence for $X$ and set an upper limit on the product branching ratio for $K_L^0->pi0pi0X$, $X->\gamma\gamma$ of $2.4 \times 10^{-7}$ at the 90% confidence level. Upper limits on the branching ratios in the mass region of $X$ from 194.3 to 219.3 MeV/$c^2$ are also presented. 
Y. C. Tung, Y. B. Hsiung, M. L. Wu, K. F. Chen, J. K. Ahn, Y. Akune,   V. Baranov, J. Comfort, M. Doroshenko, Y. Fujioka, T....