Search for the decay $K_S \to \pi^0\gamma\gamma$
  A search for the decay $K_S\to\pi^0\gamma\gamma$ has been made using the NA48 detector at the CERN SPS. Using data collected in 1999 during a 40-hour run with a high-intensity $K_S$ beam, an upper limit for the branching ratio $BR(K_S\to\pi^0\gamma\gamma, z\ge 0.2)<3.3\times\nolinebreak 10^{-7}$ has been obtained at 90% confidence level, where $z=m_{\gamma\gamma}^2/m_{K_0}^2$. 
A. Lai, et al. (NA48 Collaboration)