Upper Limit on the Branching Ratio for the Decay $\pi^0 \to \nu \bar\nu$
  A sample of kinematically identified $K^+ \to \pi^+ \pi^0$ decays obtained with the E949 detector was used to search for the helicity-suppressed decay $\pi^0 \to \nu \bar{\nu}$ resulting in an upper limit of $2.7\times10^{-7}$ at 90% confidence level.The upper limit is also applicable to $\pi^0$ decays into unknown weakly interacting particles. 
E949 Collaboration, A.V. Artamonov, et al