Study of the K0(L) --> pi0 pi0 nu nu-bar decay
  The rare decay K0(L) --> pi0 pi0 nu nu-bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10^9 K0L decays, an upper limit of 8.1 x 10^{-7} was obtained for the branching fraction at 90% confidence level. We also set a limit on the K0(L) --> pi0 pi0 X (X --> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10^{-7} to 4.0 x 10^{-5} for the mass of X ranging from 50 MeV/c^2 to 200 MeV/c^2. 
E391a Collaboration: R. Ogata, S. Suzuki, J. K. Ahn, Y. Akune, V.   Baranov, K. F. Chen, J. Comfort, M. Doroshenko, Y. Fujio...
hep-ex