We report the first observation of the radiative decay B K using a data sample of 772 10
B B pairs collected at the 4S resonance with the Belle detector at the KEKB asymmetric-energy
collider. We observe a signal of 37 events with a significance of 5.4 standard deviations including
systematic uncertainties. The measured branching fraction is BB K 274 060 032
10 where the uncertainties are statistical and systematic, respectively. We also report the first
measurements of time-dependent CP-violation parameters