Search for a light pseudoscalar particle in the decay $K^0_L \to \pi^0 \pi^0 X$ (2008)
Tung, Y. C., Hsiung, Y. B., Wu, M. L., Chen, K. F., Ahn, J. K., Akune, Y., ...
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...