Search for the decay B0 to gamma gamma
  The rare decay B0 -> gamma gamma is searched for in 104 fb^-1 of data, corresponding to 111 x 10^6 BBar pairs, collected with the Belle detector at the KEKB asymmetric-energy e+ e- collider. No evidence for the signal is found, and an upper limit of 6.2 x 10^-7 at 90% confidence level is set for the corresponding branching fraction. 
The Belle Collaboration: S. Villa, et al